Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	CTMX
Entity Registrant Name	CytomX Therapeutics, Inc.
Entity Central Index Key	0001501989
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 876.4
Entity Common Stock, Shares Outstanding		45,111,282

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 247,577	$ 177,548
Short-term investments	188,550	196,562
Accounts receivable	97	10,139
Prepaid expenses and other current assets	9,251	4,352
Total current assets	445,475	388,601
Property and equipment, net	6,934	4,218
Intangible assets, net	1,458	1,604
Goodwill	949	949
Restricted cash	917	917
Other assets	1,375	1,355
Total assets	457,108	397,644
Current liabilities:
Accounts payable	5,132	4,205
Accrued liabilities	26,724	16,382
Income tax payable	13,339	1
Deferred revenues, current portion	52,713	40,559
Total current liabilities	97,908	61,147
Deferred revenue, net of current portion	225,267	264,704
Other long-term liabilities	3,050	1,897
Total liabilities	326,225	327,748
Commitments and contingencies (Note 11)
Stockholders' equity
Common stock, $0.00001 par value; 75,000,000 shares authorized at December 31, 2018 and 2017; 45,083,209 and 38,478,560 shares issued and outstanding at December 31, 2018 and 2017, respectively	1	1
Additional paid-in capital	445,956	289,454
Accumulated other comprehensive loss	(93)	(94)
Accumulated deficit	(314,981)	(219,465)
Total stockholders' equity	130,883	69,896
Total liabilities, convertible preferred stock and stockholders' equity	457,108	397,644
Convertible Preferred Stock
Stockholders' equity
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized at December 31, 2018 and 2017; no shares issued and outstanding at December 31, 2018 and 2017, respectively

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	45,083,209	38,478,560
Common stock, shares outstanding	45,083,209	38,478,560
Convertible Preferred Stock
Convertible Preferred stock, par value	$ 0.00001	$ 0.00001
Convertible Preferred stock, shares authorized	10,000,000	10,000,000
Convertible Preferred stock, shares issued	0	0
Convertible Preferred stock, shares outstanding	0	0

STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 59,502	$ 71,623	$ 12,845
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Revenues from related parties			$ 2,198
Total revenues	$ 59,502	$ 71,623	15,043
Operating expenses:
Research and development	103,866	92,277	54,755
General and administrative	33,510	25,605	19,874
Total operating expenses	137,376	117,882	74,629
Loss from operations	(77,874)	(46,259)	(59,586)
Interest income	7,641	2,674	736
Other income (expense), net	(68)	(27)	(69)
Loss before income taxes	(70,301)	(43,612)	(58,919)
Provision for (benefit from) income taxes	14,303	(513)	(19)
Net loss	$ (84,604)	$ (43,099)	$ (58,900)
Net loss per share, basic and diluted	$ (2.03)	$ (1.16)	$ (1.63)
Shares used to compute net loss per share, basic and diluted	41,664,382	37,166,830	36,234,732
Other comprehensive loss:
Changes in unrealized gain (losses) on investments	$ 1	$ (67)	$ 49
Total comprehensive loss	$ (84,603)	$ (43,166)	$ (58,851)

STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Stockholder Notes	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit
Beginning balance at Dec. 31, 2015	$ 126,068	$ 1	$ (78)	$ 243,687	$ (76)	$ (117,466)
Beginning balance, shares at Dec. 31, 2015		36,033,209
Exercise of stock options	$ 643			643
Exercise of stock options, shares	414,396	414,396
Issuance of common stock under the Employee Stock Purchase Plan	$ 287			287
Issuance of common stock under the Employee Stock Purchase Plan, shares		31,564
Issuance of common stock in connection with services	159			159
Issuance of common stock in connection with services, shares		11,000
Repayment on stockholders note	78		$ 78
Stock-based compensation	10,095			10,095
Other comprehensive income (loss)	49				49
Net loss	(58,900)					(58,900)
Ending balance at Dec. 31, 2016	78,479	$ 1		254,871	(27)	(176,366)
Ending balance, shares at Dec. 31, 2016		36,490,169
Exercise of stock options	$ 3,165			3,165
Exercise of stock options, shares	764,576	764,576
Issuance of common stock under the Employee Stock Purchase Plan	$ 674			674
Issuance of common stock under the Employee Stock Purchase Plan, shares	67,746	67,746
Issuance of common stock pursuant to the Amgen Stock Purchase Agreement	$ 19,457			19,457
Issuance of common stock pursuant to the Amgen Stock Purchase Agreement, shares		1,156,069
Stock-based compensation	11,287			11,287
Other comprehensive income (loss)	(67)				(67)
Net loss	(43,099)					(43,099)
Ending balance at Dec. 31, 2017	69,896	$ 1		289,454	(94)	(219,465)
Ending balance, shares at Dec. 31, 2017		38,478,560
Impact of adoption of new accounting pronouncements	(10,912)					(10,912)
Issuance of common stock in follow-on offering, net of issuance costs	134,596			134,596
Issuance of common stock in follow-on offering, net of issuance costs, shares		5,867,347
Exercise of stock options	$ 4,156			4,156
Exercise of stock options, shares	673,382	673,382
Issuance of common stock under the Employee Stock Purchase Plan	$ 872			872
Issuance of common stock under the Employee Stock Purchase Plan, shares	63,920	63,920
Stock-based compensation	$ 16,878			16,878
Other comprehensive income (loss)	1				1
Net loss	(84,604)					(84,604)
Ending balance at Dec. 31, 2018	$ 130,883	$ 1		$ 445,956	$ (93)	$ (314,981)
Ending balance, shares at Dec. 31, 2018		45,083,209

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net loss	$ (84,604)	$ (43,099)	$ (58,900)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Loss/(gain) on disposal of property and equipment		17	(47)
Depreciation and amortization	1,884	1,645	1,733
Amortization of premium (accretion of discount) on investments	(1,701)	371	1,662
Stock-based compensation expense	16,878	11,287	10,095
Issuance of common stock in connection with services			159
Non-cash acquisition of in process research and development asset charged to expense		10,700
Deferred income taxes		(513)	6
Changes in operating assets and liabilities
Accounts receivable	10,042	(7,980)	(1,787)
Related party accounts receivable		154	218
Prepaid expenses and other current assets	(4,899)	(456)	(1,597)
Other assets	(20)	1,618	(2,609)
Accounts payable	261	(2,441)	1,765
Accrued liabilities and other liabilities	24,833	9,157	3,953
Deferred revenue	(38,195)	189,913	43,317
Net cash (used in) provided by operating activities	(75,521)	170,373	(2,032)
Cash flows from investing activities:
Purchases of property and equipment	(3,788)	(1,559)	(2,176)
Proceeds from sales of assets			52
Purchases of short term investments	(204,601)	(218,707)	(121,517)
Maturities of short term investments	214,315	99,000	169,500
Net cash provided by (used in) investing activities	5,926	(121,266)	45,859
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	134,596	19,957
Proceeds from employee stock purchases and exercise of stock options	5,028	3,839	930
Proceeds from stockholder notes			78
Payment of deferred offering costs			(12)
Net cash provided by financing activities	139,624	23,796	996
Net increase in cash and cash equivalents	70,029	72,903	44,823
Cash, cash equivalents and restricted cash, beginning of year	178,465	105,562	60,739
Cash, cash equivalents and restricted cash, end of year	248,494	178,465	105,562
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 1,027	$ 361	$ 473

Description of the Business

12 Months Ended

Dec. 31, 2018

Disclosure Text Block [Abstract]

Description of the Business

1. Description of the Business

CytomX Therapeutics, Inc. (the “Company”) is a clinical-stage, oncology-focused biopharmaceutical company with a vision of transforming lives with safer, more effective therapeutics. The Company is pioneering a novel class of investigational antibody therapeutics, based on its Probody™ therapeutic technology platform, for the treatment of cancer. The Probody therapeutic approach is designed to more specifically target antibody therapeutics to the tumor microenvironment and minimize drug activity in healthy tissue and in circulation. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010.

Stock Offering

In July 2018, the Company completed an underwritten public offering of 5,867,347 shares of common stock at a price of $24.50 per share, which included 765,306 shares issued pursuant to the underwriters’ exercise of their option to purchase additional shares of common stock. The aggregate net proceeds received by the Company from the offering were approximately $134.6 million after deducting underwriting discounts and commissions and offering expenses of $9.2 million.

Private Placement

On September 29, 2017, the Company and Amgen entered into the Purchase Agreement, pursuant to which the Company agreed to issue and sell to Amgen 1,156,069 shares (the “Shares”) of its common stock, for an aggregate cash purchase price of $20 million. The Shares were issued and sold to Amgen at a price per share of $17.30, using a calculation method of 20-day Volume Weighted Average Price (VWAP). The Closing of the sale and issuance of the Shares, including the delivery of the aggregate purchase price, occurred on October 4, 2017.

Basis of Presentation and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation and Summary of Significant Accounting Policies

2. Basis of Presentation and Summary of Significant Accounting Policies

Basis of Presentation

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

The preparation of the financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and reported amounts of revenues and expenses during the reporting periods. Actual results could differ from those estimates.

Concentration of Credit Risk and Other Risks and Uncertainties

The Company is subject to a number of risks similar to other biopharmaceutical companies in the early stage, including, but not limited to, the need to obtain adequate additional funding, possible failure of preclinical testing or clinical trials, the need to obtain marketing approval for its product candidates, competitors developing new technological innovations, the need to successfully commercialize and gain market acceptance of the Company’s products, and protection of proprietary technology. If the Company does not successfully obtain regulatory approval, commercialize or partner any of its product candidates, it will be unable to generate revenue from product sales or achieve profitability.

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents, short-term investments and accounts receivable. Substantially all the Company’s cash is held by one financial institution. Such deposits may, at times, exceed federally insured limits. The Company invests its cash equivalents and short-term investments in highly rated money market funds and its short-term investments in U.S. Government Bonds.

Customers and collaboration partners who represent 10% or more of the Company’s total revenue during each period presented or accounts receivable balance at each respective balance sheet date are as follows (in thousands):

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2018		2017		2016		2018		2017
AbbVie Ireland Unlimited Company	$	18,997	$	19,434	$	3,268		—	**
Bristol-Myers Squibb Company		32,780		36,492		9,577		97		10,126
ImmunoGen, Inc.	*			12,503		—		—		—
Pfizer Inc.	*		*			2,198		—	**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	51,777	$	68,429	$	15,043	$	97	$	10,126

*	Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented.

**	Accounts receivable balance from the customer was less than 10% of the Company’s total accounts receivable as of the respective periods presented.

All of the Company’s customers are located in the United States of America.

Segments

Management has determined that it has one business activity and operates as one operating segment as it only reports financial information on an aggregate basis to its chief executive officer and chief financial officer, who are the Company’s chief operating decision makers. All long-lived assets are maintained in the United States of America.

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets that sum to the total of the amounts shown in the statement of cash flows (in thousands):

	December 31
	2018		2017
Cash and cash equivalents	$	247,577	$	177,548
Restricted cash		917		917
	$	248,494	$	178,465

Short-term Investments

All investments have been classified as “available-for-sale” and are carried at fair value as determined based upon quoted market prices or pricing models for similar securities at period end. Generally, those investments with contractual maturities greater than 12 months are considered long-term investments. Unrealized gains and losses, deemed temporary in nature, are reported as a component of accumulated other comprehensive income (loss), net of tax.

A decline in the fair value of any security below cost that is deemed other than temporary results in a charge to earnings and the corresponding establishment of a new cost basis for the security. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Dividend and interest income are recognized when earned. Realized gains and losses are included in earnings and are derived using the specific identification method for determining the cost of securities sold.

Property and Equipment, net

Property and equipment are recorded at cost net of accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the respective assets. The useful lives of property and equipment are as follows:

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.

Goodwill and Intangible Assets

Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible assets acquired in business combinations. Goodwill and other intangible assets with indefinite lives are not amortized, but are assigned to reporting units and tested for impairment annually, or whenever there is an impairment indicator. Intangible assets are comprised of in-process research and development. The Company assesses impairment indicators annually as of December 31 or more frequently, if a change in circumstances or the occurrence of events suggests the remaining value may not be recoverable. Intangible assets that are not deemed to have an indefinite life are amortized over their estimated useful lives. There was no impairment of goodwill or intangible assets identified during the years ended December 31, 2018, 2017 and 2016.

Impairment of Long-Lived Assets

Long-lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset (or asset group) may not be recoverable and prior to any goodwill impairment test. An impairment loss is recognized when the total of estimated undiscounted future cash flows expected to result from the use of the asset (or asset group) and its eventual disposition is less than its carrying amount. Impairment, if any, would be assessed using discounted cash flows or other appropriate measures of fair value. There was no impairment of long-lived assets during the years ended December 31, 2018, 2017 and 2016.

Revenue Recognition

On January 1, 2018, the Company adopted Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606), which supersedes the revenue recognition requirements in Accounting Standards Codification (ASC) Topic 605, Revenue Recognition (ASC 605), using the modified retrospective transition method applied to those contracts which were not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period amounts have not been adjusted and continue to be reported in accordance with our historic accounting under ASC 605.

The Company’s revenues are primarily derived through its license, research, development and commercialization agreements. The terms of these types of agreements may include (i) licenses for the Company’s technology or programs, (ii) research and development services, and (iii) services or obligations in connection with participation in research or steering committees. Payments to the Company under these arrangements typically include one or more of the following: nonrefundable upfront and license fees, research funding, milestone and other contingent payments to the Company for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. In the event that the Company receives non-cash consideration such as in the form of a research license and research support services from the counterparty, under ASC 606, the transaction price of a non-monetary exchange that has commercial substance is estimated based on the fair value of the non-cash consideration received, which may be determined through a valuation analysis.

The Company assesses whether the promises in its arrangements with customers are considered distinct performance obligations that should be accounted for separately. Judgment is required to determine whether the license to the Company’s intellectual property is distinct from the research and development services or participation on steering committees.

The transaction price in each arrangement is allocated to the identified performance obligations based on the standalone selling price (“SSP”) of each distinct performance obligation, which requires judgment. In instances where SSP is not directly observable, such as when a license or service is not sold separately, SSP is determined using information that may include market conditions and other observable inputs. Due to the early stage of the Company’s licensed technology, the license of such technology is typically combined with research and development services and steering committee participation as one performance obligation. The Company’s performance creates an asset that does not have an alternative use to the customer and the Company has an enforceable right to payment at all times for performance completed to date. In these cases, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue. The Company evaluates the measure of progress each reporting period and, if necessary, adjusts the measure of performance and related revenue recognition.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones. Such payments are typically payable under the collaborations when the collaboration partner claims or selects a target, or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, or upon receipt of actual marketing approvals of a covered product or for additional indications. At each reporting date, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price by using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. Once determined, the transaction price is allocated to each performance obligation on a relative SSP basis. At the end of each subsequent reporting period, the Company re-evaluates the probability or achievement of each such milestone and any related constraint, and if necessary, adjusts its estimate of the overall transaction price.

The Company’s collaboration and license agreements may also include contingent payments related to sales-based milestones. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Sales-based milestones are recognized at the later of when the associated performance obligation has been satisfied or when the sales occur. Unlike other contingency payments, such as regulatory milestones, sales-based milestones are not included in the transaction price based on estimates at the inception of the contract, but rather, are included when the sales or usage occur.

AbbVie Ireland Unlimited Company (“AbbVie”), one of the Company’s collaboration partners, entered into a license agreement with Seattle Genetics, Inc. (“SGEN”) to license certain intellectual property rights. As part of the Company’s collaboration agreement with AbbVie, the Company pays SGEN sublicense fees. These sublicense fees are treated as reductions to the transaction price and combined with the performance obligation to which they relate. Milestone payments, when considered probable of being reached and when a significant revenue reversal would not be probable of occurring, are also recorded net of the associated sublicense fees and included in the transaction price.

Comprehensive Loss

Comprehensive loss represents all changes in stockholders’ equity except those resulting from distributions to stockholders. The Company’s unrealized gains and losses on short-term investments represent the only component of other comprehensive income (loss) that is excluded from the reported net loss.

Contract Balances

Customer payments are recorded as deferred revenue upon receipt or when due and may require deferral of revenue recognition to a future period until the Company performs its obligations under these arrangements. Amounts payable to the Company are recorded as accounts receivable when the Company’s right to consideration is unconditional.

Stock-Based Compensation

The Company measures its stock-based awards made to employees based on the fair values of the awards as of the grant date using the Black-Scholes option-pricing model. Stock-based compensation expense is recognized over the requisite service period using the ratable method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. Prior to 2017, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised in subsequent periods if actual forfeitures differ from those estimates. After the adoption of ASU No. 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting (“ASU 2016-09”) in 2017, forfeitures are recognized as they occur.

Stock-based compensation expense for options granted to non-employees as consideration for services received is measured on the date of performance at the fair value of the consideration received or the fair value of the equity instruments issued, using the Black-Scholes option-pricing model, whichever can be more reliably measured. Compensation expense for options granted to non- employees is periodically remeasured as the underlying options vest.

Income Taxes

The Company accounts for income taxes using an asset and liability approach. Deferred tax assets and liabilities reflect the net tax effects of temporary differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The Company records a valuation allowance to reduce its deferred tax assets to reflect the net amount that it believes as more likely than not to be realized. Realization of the deferred tax assets is dependent on the generation of future taxable income, the amount and timing of which are uncertain. The valuation allowance requires an assessment of both positive and negative evidence when determining whether it is more likely than not that deferred tax assets are recoverable. Based upon the weight of available evidence at December 31, 2018, the Company continues to maintain a full valuation allowance against all of its deferred tax assets after management considered all available evidence, both positive and negative, including but not limited to its historical operating results, income or loss in recent periods, cumulative income in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts.

The Company recognizes the tax effects of an uncertain tax position only if it is more likely than not that it will be sustained based solely on its technical merits as of the reporting date and only in an amount more likely than not that it will be sustained upon review by the tax authorities. The Company evaluates uncertain tax positions on a quarterly basis and adjust the liability for changes in facts and circumstances, such as new regulations or interpretations by the taxing authorities, new information obtained during a tax examination, significant amendment to an existing tax law, or resolution of an examination. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences will impact the income tax provision in the period in which such determination is made. The resolution of its uncertain income tax positions is dependent on uncontrollable factors such as law changes, new case law, and the willingness of the income tax authorities to settle, including the timing thereof and other factors. Although the Company does not anticipate significant changes to its uncertain income tax positions in the next twelve months, items outside of its control could cause its uncertain income tax positions to change in the future, which would be recorded in its statements of operations. Interest and/or penalties related to income tax matters are recognized as a component of income tax expense.

Accrued Research and Development Costs

The Company records accrued liabilities for estimated costs of research and development activities conducted by third-party service providers, which include the conduct of preclinical and clinical studies, and contract manufacturing activities. The Company records the estimated costs of research and development activities based upon the estimated amount of services provided but not yet invoiced, and includes these costs in accrued liabilities in the balance sheets and within research and development expense in the statements of operations. These costs are a significant component of the Company’s research and development expenses. The Company accrues for these costs based on factors such as estimates of the work completed and in accordance with agreements established with its third-party service providers under the service agreements. The Company makes significant judgments and estimates in determining the accrued liabilities balance in each reporting period. As actual costs become known, the Company adjusts its accrued liabilities. The Company has not experienced any material differences between accrued costs and actual costs incurred. However, the status and timing of actual services performed may vary from the Company’s estimates, resulting in adjustments to expense in future periods. Changes in these estimates that result in material changes to the Company’s accruals could materially affect the Company’s results of operations.

Research and development expenses include costs directly attributable to the conduct of research and development programs, including the cost of salaries, payroll taxes, employee benefits, materials, supplies, depreciation on and maintenance of research equipment, the cost of services provided by outside contractors, and the allocated portions of facility costs, such as rent, utilities, insurance, repairs and maintenance, depreciation, and general support services. All costs associated with research and development are expensed as incurred.

Reclassifications

Certain reclassifications have been made to prior period amounts to conform to current period presentation. Such reclassifications have no effect on revenues, loss from operations or net loss as previously reported.

Adopted and Recent Accounting Pronouncements

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Adopted and Recent Accounting Pronouncements

3. Adopted and Recent Accounting Pronouncements

Adopted Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606), which requires an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. ASU 2014-09 replaced most existing revenue recognition guidance in U.S. GAAP. The standard permits the use of either the retrospective or cumulative effect transition method. Additionally, in March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarifies the implementation guidance on principal versus agent considerations in ASU No. 2014-09. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies certain aspects of identifying performance obligations and licensing implementation guidance. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which relates to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. These standards (collectively Accounting Standard Codification Topic 606 (“ASC 606”) have the same effective date and transition date of January 1, 2018. The Company adopted ASC 606 on January 1, 2018, using the modified retrospective transition method. The Company elected to use the practical expedient for contract modifications whereby the aggregate effect of all modifications that occurred prior to the transition date can be reflected when identifying performance obligations and determining and allocating the transaction price.

The Company evaluated its contracts with customers under ASC 606. The impact of adopting ASC 606 on the Company’s results of operations, financial condition, and cash flows varies depending on the contract. The Company recorded adjustments upon the adoption of ASC 606 as a result of the different accounting treatment of its revenue agreements with respect to the inclusion of milestone payments in the initial transaction price and the method to be used to recognize upfront fees. Under the prior revenue recognition standard, milestone payments were recognized when earned and upfront fees were generally recognized as revenue over the research term on a straight-line basis if another method of revenue recognition did not more clearly match the pattern of delivery of goods or services to the customer. Under ASC 606, milestone payments are included in the initial transaction price when it is probable that a significant reversal of the milestone payment will not occur. In addition, the Company can no longer default to the straight-line method as the default method in recognizing revenue for goods or services delivered over time. As such, the amount and timing of revenue recognition for its collaboration agreements changed under ASC 606. The impact of the adoption of ASC 606 was an increase in the balance of deferred revenue and an increase in the accumulated deficit balance of $10.9 million on January 1, 2018 in the Company’s Balance Sheet.

The following table summarizes the impact of adopting ASC 606 on select balance sheet line items (in thousands):

	As of December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Liabilities
Income tax payable	$	14,886		$	(1,547	)	$	13,339
Other long-term liabilities		3,249			(199	)		3,050
Deferred revenue - current		45,288			7,425			52,713
Deferred revenue - long-term		214,790			10,477			225,267
Stockholders' Equity
Accumulated deficit		(298,825	)		(16,156	)		(314,981	)

	Three Months Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	10,022		$	1,449		$	11,471
Loss from Operations		(27,260	)		1,449			(25,811	)
Loss before income taxes		(24,770	)		1,449			(23,321	)
Provision for income taxes		11,566			(2,654	)		8,912
Net loss		(36,336	)		4,103			(32,233	)
Net loss per share, basic and diluted		(0.81	)		0.09			(0.72	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	65,966		$	(6,464	)	$	59,502
Loss from Operations		(71,410	)		(6,464	)		(77,874	)
Loss before income taxes		(63,837	)		(6,464	)		(70,301	)
Provision for income taxes		16,049			(1,746	)		14,303
Net loss		(79,886	)		(4,718	)		(84,604	)
Net loss per share, basic and diluted		(1.92	)		(0.11	)		(2.03	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Cash flows from operating activities:
Net loss	$	(79,886	)	$	(4,718	)	$	(84,604	)
Changes in operating assets and liabilities:
Accrued liabilities, income tax payable and other long-term liabilities		26,579			(1,746	)		24,833
Deferred revenue		(44,659	)		6,464			(38,195	)

In August 2016, the FASB issued ASU No. 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The new standard provides clarification on the cash flow presentation and classification of certain transactions, including debt prepayment or extinguishment, settlement of certain debt instruments, contingent consideration payments made after a business combination, proceeds from the settlement of certain insurance claims and distributions received from equity method investees. The Company adopted this standard in its first quarter ended March 31, 2018. The adoption of this standard had no significant impact on the Company’s financial statements.

In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash, Statement of Cash Flows (Topic 230). ASU 2016-18 requires that a statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. The Company adopted this standard in its first quarter ended March 31, 2018. The Company has revised the presentation of restricted cash in its Statements of Cash Flows and provided the additional disclosures required under this standard.

In May 2017, the FASB issued ASU No. 2017-09, Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting. This accounting standard update provides clarity when a change to terms or conditions of a share-based payment award must be accounted for as a modification. The new guidance requires modification accounting if the vesting condition, fair value or the award classification is not the same both before and after a change to the terms and conditions of the award. The Company adopted this standard on January 1, 2018. The adoption of this standard did not impact on the Company’s financial position or results of operations for the year ended December 31, 2018.

In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 (SEC Update). This standard adds various SEC paragraphs pursuant to the issuance of SEC Staff Accounting Bulletin No. 118, which clarifies the SEC Staff’s views on income tax accounting implications of the Tax Cuts and Jobs Act (the “Tax Act”). It requires reporting of provisional amounts for specific income tax effects of the Tax Act for which the accounting under ASC Topic 740 will be incomplete, but a reasonable estimate can be determined. For provisional amounts for income tax effects of the Tax Act for which a reasonable estimate cannot be determined, ASC Topic 740 should be applied based on provisions of the tax laws that were in effect immediately prior to the Tax Act being enacted. Provisional amounts for income tax effects for which a reasonable estimate cannot be determined would be reported in the first reporting period in which a reasonable estimate can be determined. The provisional amounts during a measurement period do not extent beyond one year of the enactment date. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act. In the fourth quarter of 2018, the Company completed the analysis to determine the effect of the Tax Act. No material adjustments were recorded as of December 31, 2018.

Recent Accounting Pronouncements

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), that amends the guidance for the accounting and disclosure of leases. This new standard requires that lessees recognize on the balance sheet the assets and liabilities that arise from leases, including leases classified as operating leases under current GAAP, and disclose qualitative and quantitative information about leasing arrangements. The new standard requires a modified retrospective approach to adoption and is effective for interim and annual periods beginning after December 15, 2018 but may be adopted earlier. In July 2018, the FASB further amended this standard to allow for a new transition method that provides the option to use the effective date as the date of initial application. The Company will adopt this standard using the modified retrospective approach on the effective date of January 1, 2019.

The Company has elected the package of practical expedients permitted under ASC 842. Accordingly, the Company continues to account for its existing operating leases as operating leases under the new guidance, without reassessing (a) whether the contracts contain a lease under ASC 842, (b) whether classification of the operating leases would be different in accordance with ASC 842, or (c) whether the unamortized initial direct costs before transition adjustments would have met the definition of initial direct costs in ASC 842 at lease commencement. In addition, the Company also elected the short-term lease practical expedients allowed under the standard. As a result of the adoption of ASC 842, the Company expects to recognize on January 1, 2019 on its balance sheet a right-of-use asset and a lease liability of approximately $27 million to $32 million. This standard will not have material impact on the Company’s results of operations or cash flows.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. The new standard changes the impairment model for most financial assets and certain other instruments. Under the new standard, entities holding financial assets and net investment in leases that are not accounted for at fair value through net income are to be presented at the net amount expected to be collected. An allowance for credit losses will be a valuation account that will be deducted from the amortized cost basis of the financial asset to present the net carrying value at the amount expected to be collected on the financial asset. The new standard will be effective for the Company on January 1, 2020. The Company is currently evaluating the impact of this new guidance.

In January 2017, the FASB issued ASU No. 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. The new standard simplifies the measurement of goodwill by eliminating the Step 2 impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. The new guidance requires an entity to compare the fair value of a reporting unit with its carrying amount and recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value. Additionally, an entity should consider income tax effects from any tax-deductible goodwill on the carrying amount of the reporting unit when measuring the goodwill impairment loss, if applicable. The new guidance becomes effective for goodwill impairment tests in fiscal years beginning after December 15, 2019, though early adoption is permitted. The Company is currently evaluating the impact of this new guidance.

In February 2018, the FASB issued ASU No. 2018-02, Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. The amendments in this standard allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. The new standard will be effective for the Company on January 1, 2019. The Company does not expect the adoption of this ASU will have a material impact on its financial statements.

In June 2018, the FASB issued ASU No. 2018-07, Compensation - Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. The amendments in this ASU expand the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. This new guidance is effective for the Company on January 1, 2019 with early adoption permitted. The Company does not expect the adoption of this ASU will have a material impact on its financial statements.

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820). The amendments in this ASU modify the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement. Various disclosure requirements have been removed, including the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, the valuation processes for Level 3 fair value measurements held at the end of the reporting period. The ASU also modified various disclosure requirements and added some disclosure requirements for Level 3 fair value measurements. The amendments in this ASU are effective for the Company on January 1, 2020. The additional disclosures on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this ASU and delay adoption of the additional disclosures until their effective date. The Company does not expect the adoption of this ASU will have a material impact on its financial statements.

In August 2018, the FASB issued ASU No. 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40). The amendments in this ASU on the accounting for implementation, setup and other upfront costs (collectively “implementation costs”) apply to entities that are a customer in a hosting arrangement. The amendments under this ASU align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. Accordingly, the amendments in this ASU require an entity in a hosting arrangement that is a service contract to follow the guidance in Subtopic 350-40 to determine which implementation costs to expense. They also require an entity to expense the capitalized implementation costs of a hosting arrangement that is a service contract over the term of the hosting arrangement. This ASU is effective for the Company on January 1, 2020. The Company is currently evaluating the impact of this new guidance.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the interaction between Topic 808 and Topic 606. The amendments in this ASU targeted improvements to generally accepted accounting principles for collaborative arrangements by clarifying that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. In addition, unit-of-account guidance in Topic 808 was aligned with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. The ASU is effective for the Company on January 1, 2020, and interim periods within those fiscal years. Early adoption is permitted. The Company is currently evaluating the impact of this new guidance.

Net Loss Per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

4. Net Loss Per Share Attributable to Common Stockholders

Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding for the period. Diluted net loss per share attributable to common stockholders is calculated using the weighted-average number of common shares outstanding, plus potential dilutive common stock during the period. Diluted net loss per share attributable to common stockholders is the same as basic net loss per share attributable to common stockholders since the effect of the potentially dilutive securities is anti-dilutive.

The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share as their effect would have been anti-dilutive:

	Year Ended December 31,
	2018		2017		2016
Options to purchase common stock		7,478,755		6,891,123		6,086,939

Fair Value Measurements and Short-term Investments

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements and Short-Term Investments

5. Fair Value Measurements and Short-Term Investments

In accordance with Accounting Standards Codification (“ASC”) 820-10, Fair Value Measurements and Disclosures, the Company determines the fair value of financial and non-financial assets and liabilities using the fair value hierarchy, which establishes three levels of inputs that may be used to measure fair value, as follows:

•

Level I: Inputs which include quoted prices in active markets for identical assets and liabilities.

•

Level II: Inputs other than Level I that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

•

Level III: Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The carrying amounts of the Company’s financial instruments, including restricted cash, accounts receivable, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. The Company’s financial instruments consist of Level I and II assets. Level I assets consist primarily of highly liquid money market funds, some of which is included in restricted cash and U.S. government bonds that are included in short-term investments.

The following tables set forth the fair value of the Company’s financial instruments subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands):

		December 31, 2018
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	226,979	$	—	$	—		$	226,979
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		188,616		—		(66	)		188,550
Total Securities		$	416,512	$	—	$	(66	)	$	416,446

		December 31, 2017
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	164,440	$	—	$	—		$	164,440
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		196,629		—		(67	)		196,562
Total Securities		$	361,986	$	—	$	(67	)	$	361,919

No securities have contractual maturities of longer than one year.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Property and Equipment

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment

6. Property and Equipment

Property and equipment, net consisted of the following (in thousands):

	December 31
	2018			2017
Machinery and equipment	$	10,498		$	7,162
Computer equipment and software		955			897
Furniture and fixtures		749			643
Leasehold improvements		893			701
Construction in progress		785			23
		13,880			9,426
Less: accumulated depreciation and amortization		(6,946	)		(5,208	)
	$	6,934		$	4,218

Depreciation and amortization expense was $1.7 million, $1.5 million and $1.7 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

7. Goodwill and Intangible Assets

Goodwill and in-process research and development assets resulted from a series of integrated financing transactions in 2010 that was accounted for as a business combination. The in-process research and development relates to the Company’s proprietary Probody Platform and was accounted for as an indefinite-lived intangible asset until the underlying project was completed or abandoned. In connection with the collaboration agreements, the Company began amortizing the intangible asset in 2017. The intangible asset is being amortized over the estimated lives of the patents which average 12 years. The amortization for the year ended December 31, 2018 and 2017 was $0.1 million and $0.1 million, respectively.

Goodwill and intangible assets consisted of the following (in thousands):

Goodwill
	December 31,
	2018			2017
Goodwill	$	949		$	949

Intangible assets
	December 31,
	2018			2017
In-process research and development	$	1,750		$	1,750
Less accumulated amortization		(292	)		(146	)
	$	1,458		$	1,604

Accrued Liabilities

12 Months Ended

Dec. 31, 2018

Accrued Liabilities Current [Abstract]

Accrued Liabilities

8. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2018		2017
Research and clinical expenses	$	18,520	$	10,068
Payroll and related expenses		6,585		4,526
Legal and professional expenses		830		1,523
Other accrued expenses		789		265
Total	$	26,724	$	16,382

Research and Collaboration Agreements

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Research and Collaboration Agreements

9. Research and Collaboration Agreements

The following table summarizes the revenue by collaboration partner (in thousands):

	Year Ended December 31,
	2018		2017		2016
AbbVie	$	18,997	$	19,434	$	3,268
Amgen		4,899		1,311		—
BMS		32,780		36,492		9,577
ImmunoGen		1,471		12,503		—
Pfizer		1,355		1,883		2,198
Total Revenue	$	59,502	$	71,623	$	15,043

AbbVie Ireland Unlimited Company

In April 2016, the Company and AbbVie entered into two agreements, a CD71 Co-Development and Licensing Agreement (the “CD71 Agreement”) and a Discovery Collaboration and Licensing Agreement (the “Discovery Agreement” and together with the CD71 Agreement the “AbbVie Agreements”). Under the terms of the CD71 Agreement, the Company and AbbVie will co-develop a Probody Drug Conjugates (“PDC”) against CD71, with the Company responsible for pre-clinical and early clinical development. AbbVie will be responsible for later development and commercialization, with global late-stage development costs shared between the two companies. The Company will assume 35% of the net profits or net losses related to later development unless it opts-out. If the Company opts-out from participation of co-development of the CD71 PDC, AbbVie will have sole right and responsibility for the further development, manufacturing and commercialization of such CD71 PDC.

Under the CD71 Agreement, the Company received an upfront payment of $20.0 million in April 2016, and is eligible to receive up to $470.0 million in development, regulatory and commercial milestone payments, a 35% profit split on U.S. sales, and royalties on ex-U.S. sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reversion to a royalty on U.S. sales, and reduction in royalties on ex-U.S. sales, if the Company opts-out from the co-development of the CD71 PDC. The Company’s share of later stage co-development costs for each CD71 PDC are capped, provided that AbbVie may offset the Company’s co-development cost above the capped amounts from future payments such as milestone payments and royalties. In July 2017, the Company received a milestone payment of $14.0 million (net of payment of an associated sublicense fee of $1.0 million to SGEN under the Seattle Genetics Agreement) from AbbVie for achieving certain milestones required to be met to begin GLP toxicology studies under the CD71 Agreement. In May 2018, the United States Food and Drug Administration (“FDA”) cleared the IND application for CX-2029, the PDC targeting CD71. As a result, the Company achieved the IND success criteria under the CD71 Agreement and received a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). The Company commenced enrollment of our Phase 1/2 clinical trial and dosed the first patient in a clinical trial at the end of the second quarter of 2018.

Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDCs against up to two targets, one of which was selected in March 2017. The Company shall perform research services to discover the Probody therapeutics and create PDCs for the nominated collaboration targets. From that point, AbbVie shall have sole right and responsibility for development and commercialization of products comprising or containing such PDCs (“Discovery Licensed Products”).

Under the Discovery Agreement, the Company received an upfront payment of $10.0 million in April 2016 and may receive an additional payment upon the selection by AbbVie of the second target and the satisfaction of certain success criteria under the CD71 Agreement. AbbVie has not selected the second target, but the success criteria under the CD71 Agreement were met in September 2016. The Company is also eligible to receive up to $275.0 million in target nomination, development, regulatory and commercial milestone payments and royalties in the high single to low teens from commercial sales of any resulting PDCs.

The Company has determined that the CD71 and Discovery Agreements with AbbVie should be combined and evaluated as a single arrangement in determining revenue recognition, because both agreements were concurrently negotiated and executed.

The Company identified the following performance obligations at the inception of the AbbVie Agreements:

(1) the research, development and commercialization license for CD71 Probody therapeutic,

(2) the research services related to CD71 Probody therapeutic,

(3) the obligation to participate in the CD71 Agreement joint research committee,

(4) the research services related to the first discovery target

(5) the research, development and commercialization license for the first discovery target, and

(6) the obligation to participate in the Discovery Agreement joint research committee.

The Company concluded that, at the inception of the agreement, AbbVie’s option for the second discovery target is not a material right and is therefore not a performance obligation.

The Company determined that the research, development and commercialization licenses for CD71 and discovery targets are not distinct from the Company’s respective research services and expertise. The Company considered factors such as novelty of the Probody therapeutic and PDC technology and lack of other parties’ expertise in this space, the Company’s rights to technology relating to a proprietary platform to enable the Probody therapeutic development and AbbVie’s contractual obligation to use the Company’s research services. The Company determined that the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee were a combined performance obligation and were distinct from the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. Therefore, the Company concluded that there are two distinct performance obligations:

(1)

the CD71 Agreement performance obligation consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and

(2)

the Discovery Agreement performance obligation consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee.

The total transaction price upon adoption of ASC 606 on January 1, 2018 of $39.8 million consists of $30.0 million in upfront payments, $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) less $4.2 million of estimated sublicense fees. The upfront payments under the AbbVie Agreements are allocated between the two performance obligations based on the estimated relative standalone selling prices. The $30.0 million of upfront payments is allocated $20.0 million to the CD71 Agreement, with the remaining $10.0 million allocated to the Discovery Agreement. The $14.0 million milestone payment received (net of the payment of an associated sublicense fee of $1.0 million to SGEN) and estimated sublicense fees are allocated to the CD71 Agreement performance obligation as they are directly related to the development of the CD71 Probody therapeutic.

In May 2018, the Company earned a $21.0 million milestone payment (net of the payment of an associated sublicense fee of $4.0 million to SGEN). The $21.0 million milestone payment was included as part of the transaction price in May 2018 and a revenue adjustment of $9.9 million was recognized in the second quarter of 2018 reflecting the percentage completed to-date on the project related to this milestone. The Company determined that the remaining potential milestone payments are probable of significant revenue reversal as their achievement is highly dependent on factors outside the Company’s control. Therefore, these payments have been fully constrained and were not included in the transaction price as of December 31, 2018.

Under the UCSB Agreement, the Company is obligated to make royalty payments to UCSB equal to 5% of certain sublicense revenue payments owed to or received by the Company. As of December 31, 2018 and December 31, 2017, the Company accrued sublicense fees of $1.1 million and $0.5 million, respectively, under the CD71 Agreement.

The Company recognized the initial transaction price upon adoption of ASC 606 on January 1, 2018 of $29.8 million allocated to the CD71 Agreement performance obligation using a cost-based input measure. In applying the cost-based input method, revenue is recognized based on actual full time employee (“FTE”) hours incurred as a percentage of total estimated FTE hours as the Company completes its combined performance obligation over the five-year service period.

As the Discovery Agreement performance obligation represents an obligation to continuously make the Company’s Probody therapeutic technology platform available to AbbVie, the initial transaction price of $10.0 million allocated to this performance obligation is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five years.

The Company recognized revenue of $19.0 million, $19.4 million and $3.3 million for 2018, 2017 and 2016, respectively, related to the AbbVie Agreements. As of December 31, 2018 and 2017, deferred revenue related to the CD71 Agreement performance obligation was $23.2 million and $11.2 million, respectively, and deferred revenue related to the Discovery Agreement performance obligation was $4.7 million and $6.8 million, respectively. As of both December 31, 2018 and 2017, no amount was due from AbbVie under the CD71 and Discovery Agreements.

Amgen, Inc.

On September 29, 2017, the Company and Amgen, Inc. (“Amgen”) entered into a Collaboration and License Agreement (the “Amgen Agreement”). Pursuant to the Amgen Agreement, the Company received an upfront payment of $40.0 million in October 2017. Concurrent with the entry into the Amgen Agreement, the Company and Amgen entered into a Share Purchase Agreement (the “Purchase Agreement”) pursuant to which Amgen purchased 1,156,069 shares of the Company’s common stock at a price of $17.30 per share (calculated based on a 20-day volume-weighted average price), for total proceeds of $20.0 million, which the Company received on October 6, 2017, the closing date of the transaction. The Company estimated a premium on the stock sold to Amgen of $0.5 million, which takes into account a discount due to the lack of marketability resulting from the six-month lockup period.

Under the terms of the Amgen Agreement, the Company and Amgen will co-develop a Probody T-cell engaging bi-specific therapeutic targeting EGFR (“EGFR Products”). The Company is responsible for early-stage development of EGFR Products and all related costs up to certain pre-set costs and certain limits based on clinical study size. Amgen will be responsible for late-stage development, commercialization, and all related costs of EGFR Products. Following early-stage development, the Company will have the right to elect to participate financially in the global co-development of EGFR Products with Amgen, during which the Company would bear certain of the worldwide development costs for EGFR Products and Amgen would bear the rest of such costs (the “EGFR Co-Development Option”). If the Company exercises its EGFR Co-Development Option, the Company will share in somewhat less than 50% of the profit and losses from sales of such EGFR Products in the U.S., subject to certain caps, offsets, and deferrals. If the Company chooses not to exercise its EGFR Co-Development Option, the Company will not bear any costs of later stage development. The Company is eligible to receive up to $455.0 million in development, regulatory, and commercial milestone payments for EGFR Products, and royalties in the low-double-digit to mid-teen percentage of worldwide commercial sales, provided that if the Company exercises its EGFR Co-Development option, it shall receive a profit and loss split of sales in the U.S. and royalties in the low-double-digit to mid-teen percentage of commercial sales outside of the U.S.

Amgen also has the right to select a total of up to three targets, including the two additional targets discussed below. The Company and Amgen collaborate in the research and development of Probody T-cell engaging bi-specifics products directed against such targets. Amgen has selected one such target (the “Amgen Other Product”). If Amgen exercises its option within a specified period of time, it can select two such additional targets (the “Amgen Option Products” and, together with the Amgen Other Product, the “Amgen Products”). Except with respect to preclinical activities to be conducted by CytomX, Amgen will be responsible, at its expense, for the development, manufacture, and commercialization of all Amgen Products. If Amgen exercises all of its options and advances all three of the Amgen Products, CytomX is eligible to receive up to $950.0 million in upfront, development, regulatory, and commercial milestones and tiered high single-digit to low-teen percentage royalties. The Company concluded that, at the inception of the agreement, Amgen’s option to select the two additional targets is not a material right and does not represent a performance obligation of the agreement.

At the initiation of the collaboration, CytomX had the option to select, from programs specified in the Amgen Agreement, an existing pre-clinical stage T-cell engaging bispecific product from the Amgen pre-clinical pipeline. In March 2018, CytomX selected the program. CytomX is responsible, at its expense, for converting this program to a Probody T-cell engaging bispecific product, and thereafter, will be responsible for development, manufacturing, and commercialization of the product (“CytomX Product”). Amgen is eligible to receive up to $203.0 million in development, regulatory, and commercial milestone payments for the CytomX Product, and tiered mid-single digit to low double-digit percentage royalties.

The Company considered the criteria for combining contracts in ASC 606 and determined that the Amgen Agreement and the Purchase Agreement should be combined into one contract. The Company accounted for the Amgen Agreement based on the fair values of the assets and services exchanged. The Company identified the following performance obligations at the inception of the Amgen Agreement:

(1) the research, development and commercialization license,

(2) the research and development services for the EGFR Products and the Amgen Other Product, and

(3) the obligation to participate in the joint steering committee (“JSC”) and the joint research committee (“JRC”).

The Company determined that research, development and commercialization license and the participation in the JSC and JRC are not distinct from the research and development services and therefore those performance obligations were combined into one combined performance obligation. The Amgen Other Products are accounted for as a separate performance obligation from the EGFR Products as the nature of the services being performed is not the same and the value that Amgen can derive from one program is not dependent on the success of the other.

Concurrent with the execution of the Amgen Agreement, the Company entered into a sublicense agreement whereby the Company granted Amgen a sublicense of its rights to one patent family that it co-owns with the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), that is exclusively licensed to the Company under the UCSB Agreement covering Probody antibodies and other pro-proteins in the fields of therapeutics, in vivo diagnostics and prophylactics. This sublicense was incremental to the patents, patent applications and know-how covering T-cell engaging bispecific Probody molecules that were developed and owned by the Company and licensed to Amgen. Under the UCSB Agreement, the Company is obligated to make a royalty payment to UCSB equal to 15% of certain sublicense revenue payments owed to or received by the Company. The Company determined that the calculation of the sublicense fee is not specifically addressed in the sublicense agreement when the Company simultaneously licenses the UCSB technology along with the technology the Company has developed internally. As of December 31, 2017, the Company recorded a liability of $2.1 million, which represents the Company’s best estimate of the amount to be remitted to UCSB. As of December 31, 2018, the Company determined that the estimated liability of $2.1 million was still appropriate.

The total transaction price of $51.2 million, consisting of the $40.0 million upfront payment, an estimated fair value of $10.7 million for the CytomX Product and $0.5 million of premium on the sale of the Company’s common stock, was allocated between the two performance obligations based on the relative standalone selling price of each performance obligation. To determine the standalone selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. As a result, these payments were fully constrained and were not included in the transaction price as of December 31, 2018. The Company recognizes the transaction price of $46.4 million allocated to the EGFR Products performance obligation using a cost-based input measure. In applying the cost-based input method of revenue recognition, the Company uses actual FTE hours incurred relative to estimated FTE hours expected to be incurred for the combined performance obligation. Revenue is recognized based on actual FTE hours incurred as a percentage of total estimated FTE hours as the Company completes its performance obligation over the research service period. In the fourth quarter of 2018, the JSC officially terminated any further work on two molecules due to unacceptable test results. The current plan is to evaluate other molecules as part of the candidate identification phase of the EGFR project, and as a result, there has been a change in estimate of the projected costs and research service period to seven years.

As the Amgen Other Product performance obligation represents an obligation to continuously make the Probody therapeutic technology platform available to Amgen, the initial transaction price of $4.8 million allocated to this performance obligation is recognized over the common measure of progress for the entire performance obligation over the estimated research service period of six years.

The Company recognized revenue of $4.9 million and $1.3 million for the years ended December 31, 2018 and 2017, respectively, related to the Amgen Agreement. There was no revenue recognized for the year ended December 31, 2016 because the Company entered into the agreement in 2017. As of December 31, 2018 and December 31, 2017, deferred revenue related to the EGFR Products performance obligation was $40.7 million and $45.3 million, respectively. As of December 31, 2018 and 2017, deferred revenue related to the Amgen Other Products performance obligation was $3.8 million and $4.6 million, respectively. As of December 31, 2018 and 2017, no amount was due from Amgen under the Amgen Agreement.

Bristol-Myers Squibb Company

On May 23, 2014, the Company and Bristol-Myers Squibb Company (“BMS”) entered into a Collaboration and License Agreement (the “BMS Agreement”) to discover and develop compounds for use in human therapeutics aimed at multiple immuno-oncology targets using the Company’s Probody therapeutic technology. The effective date of the BMS Agreement was July 7, 2014.

Under the terms of the BMS Agreement, the Company granted BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to four oncology targets. BMS had additional rights to substitute up to two collaboration targets within three years of the effective date of the BMS Agreement. These rights expired in May 2017. Each collaboration target has a two-year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times.

Pursuant to the BMS Agreement, the financial consideration from BMS was comprised of an upfront payment of $50.0 million and the Company was initially entitled to receive contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid-single digits to low double-digit percentages from potential future sales. The Company also receives research and development service fees based on a prescribed FTE rate that is capped.

The Company identified the following performance obligations at the inception of the BMS Agreement:

(1) the exclusive research, development and commercialization license,

(2) the research and development services and

(3) the obligation to participate in the joint research committee.

The Company determined that the license, the Company’s research services and expertise related to the development of the product candidates should be combined with the research services and participation in the joint research committee as one combined performance obligation. The Company concluded that, at the inception of the agreement, BMS’ options for the third and fourth targets were not material rights and not performance obligations. As such, each option was accounted for as a separate arrangement upon exercise. Additionally, the Company considered whether the services performed for each target should be considered separate performance obligations and concluded that all targets should be accounted for as one combined performance obligation.

The Company received an upfront payment of $50.0 million from BMS in July 2014. In January and December 2016, BMS selected the third and fourth targets, respectively, and paid the Company $10.0 million and $15.0 million, respectively, pursuant to the terms of the BMS Agreement. In December 2016, BMS selected a clinical candidate pursuant to the BMS Agreement, which triggered a $2.0 million pre-clinical milestone payment to the Company. In November 2017, the Company recognized a $10.0 million milestone payment from BMS upon approval of the investigational new drug application for the CTLA-4-directed Probody therapeutic.

On March 17, 2017, the Company and BMS entered into Amendment Number 1 to Extend Collaboration and License Agreement (the “Amendment”). The Amendment grants BMS exclusive worldwide rights to develop and commercialize Probody therapeutics for up to six additional oncology targets and two non-oncology targets. The effective date of the Amendment was April 25, 2017 (“Amendment Effective Date”). Under the terms of the Amendment, the Company continues to collaborate with BMS to discover and conduct preclinical development of Probody therapeutics against targets selected by BMS under the terms of the Amendment.

Pursuant to the Amendment, the financial consideration from BMS is comprised of an upfront payment of $200.0 million and the Company is eligible to receive up to an aggregate of $3,586.0 million as follows:

(i)

up to $116.0 million in development milestone payments per target or up to $928.0 million if the maximum of eight targets are selected for the first product modality;

(ii)

up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $992.0 million if the maximum of eight targets are selected for the first product modality;

(iii)

up to $60.0 million in sales milestone payments per target or up to $480.0 million if maximum of eight targets are selected for the first product modality; and

(iv)

up to $56.3 million in development milestone payments or up to $450.0 million if the maximum of eight targets are selected for the second product modality;

(v)

up to $62.0 million in milestone payments for the first commercial sale in various territories for up to three indications per target program or up to $496.0 million if the maximum of eight targets are selected for the second product modality;

(vi)

up to $30.0 million in sales milestone payments per target or up to $240.0 million if maximum of eight targets are selected for the second product modality.

The Company is also entitled to tiered mid-single to low double-digit percentage royalties from potential future sales. The Amendment does not change the term of the BMS’s royalty obligation under the BMS Agreement. BMS’s royalty obligation continues on a licensed-product by licensed-product basis until the later of (i) the expiration of the last claim of the licensed patents covering the licensed products in the country, (ii) the twelfth anniversary of the first commercial sale of a licensed product in a country, or (iii) the expiration of any applicable regulatory, pediatric, orphan drug or data exclusivity with respect to such product.

The initial transaction price is $272.8 million consisting of the upfront fees of $250.0 million, research and development service fees of $10.8 million and milestone payments received to date of $12.0 million. The Company determined that the remaining potential milestone payments were probable of significant revenue reversal as their achievement was highly dependent on factors outside the Company’s control. Therefore, these payments were fully constrained and were not included in the transaction price as of December 31, 2018. The BMS Agreement represents an obligation to continuously make the Probody therapeutic technology platform available to BMS. Therefore, the initial transaction price is recognized over the estimated research service period, which ends on April 25, 2023.

The Company recognized revenue of $32.8 million, $36.5 million and $9.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, deferred revenue related to the BMS Agreement was $205.6 million and $235.0 million, respectively. The amount due from BMS under the BMS Agreement was $0.1 million and $10.1 million as of December 31, 2018 and 2017, respectively.

ImmunoGen, Inc.

In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Research Agreement”). The ImmunoGen Research Agreement provided the Company with the right to use ImmunoGen’s Antibody Drug Conjugate (“ADC”) technology in combination with the Company’s Probody therapeutic technology to create a PDC directed at one specified target under a research license, and to subsequently obtain an exclusive, worldwide development and commercialization license to use ImmunoGen’s ADC technology to develop and commercialize such PDCs. The Company made no upfront cash payment in connection with the execution of the agreement. Instead, the Company provided ImmunoGen with the rights to CytomX’s Probody therapeutic technology to create PDCs directed at two targets under the ImmunoGen Research Agreement and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. In February 2016, the Company exercised its option to obtain a development and commercialization license for CX-2009 pursuant to the terms of the ImmunoGen Research Agreement (the “CX-2009 License”). In February 2017, ImmunoGen exercised its first option to obtain a development and commercialization license for one of the two targets. Substitution rights for this first target selection program terminated in February 2017 and ImmunoGen discontinued the program in July 2017. The Company recognized the remaining deferred revenue related to the discontinued program upon the termination of the program. ImmunoGen exercised its second option to obtain a development and commercialization license pursuant to the ImmunoGen Research Agreement (the “ImmunoGen 2017 License”) for a target in December 2017 and continues research work on this program.

Under the terms of the ImmunoGen Research Agreement, both the Company and ImmunoGen performed research activities on behalf of the other party for no monetary consideration through January 2018 and the arrangement was extended to June 2018, as discussed below. Each party is solely responsible for the development, manufacturing and commercialization of any products resulting from the exclusive development and commercialization license obtained by such party under the agreement.

In consideration for the ImmunoGen 2017 License, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments, up to $50.0 million in sales milestone payments and royalties in the mid-single digits percentage on the commercial sales of any resulting product. For the CX-2009 License, the Company is obligated to pay ImmunoGen up to $60.0 million in development and regulatory milestone payments and up to $100.0 million in sales milestone payments and royalties in the mid to high single digits percentage on the commercial sales of any resulting product. In August 2017, the Company made a milestone payment of $1.0 million to ImmunoGen for the first patient dosing with CX-2009. No milestone payments have been accrued to the Company under the ImmunoGen 2017 License.

The Company accounted for the ImmunoGen Research Agreement based on the fair value of the assets and services exchanged. The Company identified the following performance obligations at the inception of the ImmunoGen Research Agreement:

(1) the research license,

(2) the research services,

(3) the obligation to participate in the joint research committee,

(4) the exclusive research, development and commercialization license and

(5) the obligation to provide future technology improvements, when available.

The Company determined that the research license, the research services, the participation in the joint steering committee, and the technology improvements are not distinct from the development and commercialization license and therefore those performance obligations were combined into one combined performance obligation. The Company considered factors such as the limited economic benefits to ImmunoGen if the development and commercialization license was not obtained and the lack of sublicensing rights in the research license.

The estimated total fair value of the consideration of $13.2 million was recorded as deferred revenue at the inception of the ImmunoGen Research Agreement. In December 2017, the Company entered into the ImmunoGen 2017 License arrangement and extended the Company’s obligation to provide research services under the ImmunoGen Research Agreement to June 30, 2018. The fair value of the consideration for the combined performance obligation was recognized as revenue over the research period that ended on June 30, 2018. As of December 31, 2018, neither company has further research obligations under the ImmunoGen Research Agreement.

The estimated total fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million allocated to the research services, joint research committee participation and technology improvements, which was expensed over the period of services provided.

The Company recognized revenue of $1.5 million and $12.5 million for the year ended December 31, 2018 and 2017, respectively. There was no revenue recognized for the year ended December 31, 2016. As of December 31, 2018 and 2017, deferred revenue relating to the ImmunoGen Research Agreement was $0 and $0.7 million, respectively. As of both December 31, 2018 and 2017, no amount was due from ImmunoGen under the ImmunoGen Research Agreement.

MD Anderson

In November 2015, the Company entered into a research collaboration agreement with MD Anderson to research Probody-enabled chimeric antigen receptor killer (CAR-NK) cell therapies, known as ProCAR-NK cell therapies. Under this collaboration, MD Anderson will use the Company’s Probody technology to conduct research of ProCAR-NK cell therapies against certain targets selected by the Company in cancer immunotherapy. Under the research collaboration agreement, the Company had the right to exercise an option, during the option period expiring on November 2, 2019 and upon payment of an option exercise fee, to negotiate and acquire a worldwide, exclusive, sublicensable license from MD Anderson for development and commercialization of products directed against any of the selected targets. The research collaboration agreement continued in effect until the earlier of (i) the date that the Company exercises the option to acquire the license from MD Anderson and (ii) the expiration of the option period. The expenses related to this agreement were not material to the financial statements for the years ended December 31, 2018 and 2017. The Company decided not to exercise the option to extend the agreement and allowed it to expire on November 1, 2018.

Pfizer Inc.

In May 2013, the Company and Pfizer Inc. (“Pfizer”) entered into a Research Collaboration, Option and License Agreement (the “Pfizer Agreement”) to collaborate on the discovery and preclinical research activities related to Probody therapeutics, and PDCs for research project targets nominated by Pfizer. Pfizer nominated two research targets in 2013 and, pursuant to the Pfizer Agreement, had the option of nominating two additional research targets. In December 2014, Pfizer selected an additional research target and paid the Company $1.5 million. The option to select a fourth target lapsed in May 2016. Pfizer discontinued the epidermal growth factor receptor (“EGFR”) program and decided to terminate the remaining two targets in February and March 2018. In March 2018, Pfizer terminated the Pfizer Agreement. As such, the Company had no further performance obligations under this agreement following the termination of the Pfizer Agreement and recognized the remaining deferred revenue of $1.1 million during the first quarter of 2018.

Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6.0 million and research and development service fees based on a prescribed FTE rate per year that is capped. The Company identified the following performance obligations at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license was not distinct from the research services and participation in the joint research committee due to the specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a combined performance obligation. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target did not represent a material right and were not performance obligations.

As the combined performance obligation represented an obligation to continuously make the Probody therapeutic technology platform available to Pfizer, the initial transaction price was recognized over the common measure of progress for the entire performance obligation over the estimated research service period of five and a half years.

The Company recognized revenue of $1.4 million, $1.9 million and $2.2 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, deferred revenue relating to the Pfizer Agreement was $0 and $1.6 million, respectively. The amount due from Pfizer under the Pfizer Agreement was $0 and $13,000 as of December 31, 2018 and 2017, respectively.

Contract Liabilities

The following table presents changes in the Company’s total contract liabilities for the year ended December 31, 2018 (in thousands):

			Additions				Deductions
	Balance at 12/31/2017		ASC 606 Adoption Adjustment		Adjustment to Transaction Price from Performance Obligation Satisfied		Revenue Recognized from an Adjustment to Transaction Price During the Period			Revenue Recognized from Amounts Included in Contract Liability at the Beginning of the Period			Balance at 12/31/2018
Contract liabilities:
Deferred revenue	$	305,263	$	10,912	$	21,000	$	(11,718	)	$	(47,477	)	$	277,980

Additions of $31.9 million consists of the ASC 606 adoption adjustment of $10.9 million and $21.0 million earned (net of the payment of an associated sublicense fee of $4.0 million to SGEN) resulting from the achievement of the IND milestone under the CD71 Agreement. The $21.0 million milestone payment is included in the transaction price in May 2018 and is being recognized over the research period of the CD71 Agreement. Of the $21.0 million, $11.7 million was recognized as revenue during the year ended December 2018 based on the estimated percentage completed to-date of the CD71 project. Deductions also includes $47.5 million of revenue recognized during the year ended December 31, 2018 that was included in the contract liability balance at the beginning of the period.

The Company estimates that the $278.0 million of deferred revenue related to the following contracts as of December 31, 2018 will be recognized as revenue as set forth below. However, the timing of revenue recognition could differ from the estimates depending on facts and circumstances impacting the various contracts, including progress of research and development, resources assigned to the contracts by the Company or its collaboration partners or other factors outside of the Company’s control.

•

The $23.2 million of deferred revenue related to the CD71 Agreement as of December 31, 2018 is expected to be recognized based on actual FTE effort and program progress until approximately April 2021.

•

The $4.7 million of deferred revenue related to the Discovery Agreement as of December 31, 2018 is expected to be recognized ratably until approximately April 2021.

•

The $40.7 million of deferred revenue related to the Amgen EGFR Products as of December 31, 2018 is expected to be recognized based on actual FTE effort and program progress until approximately September 2024.

•

The $3.8 million of deferred revenue related to the Amgen Other Products as of December 31, 2018 is expected to be recognized ratably until approximately September 2023.

•

The $205.6 million of deferred revenue related to the BMS Agreement as of December 31, 2018 is expected to be recognized ratably until approximately April 2025.

License Agreement

12 Months Ended

Dec. 31, 2018

Research And Development [Abstract]

License Agreement

10. License Agreement

The Company has an exclusive, worldwide license agreement (the “UCSB Agreement”) with UCSB, relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies, and to certain patent rights the Company co-owns with UCSB covering Probody antibodies and other pro-proteins.

Pursuant to the UCSB Agreement, the Company is obligated to (i) make royalty payments to UCSB on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to UCSB upon the occurrence of certain events, (iii) make a milestone payment to UCSB upon occurrence of an IPO or change of control, and (iv) reimburse UCSB for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UCSB Agreement, it is obligated to pay UCSB a percentage of the total sublicense revenue received, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company and other permitted deductions.

In 2013, the Company amended the UCSB Agreement to reduce certain amounts due to UCSB upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to UCSB 157,332 shares of common stock. The UCSB Agreement, as amended, will remain in effect until the expiration or abandonment of the last to expire of the licensed patents.

In the years ended December 31, 2018, 2017 and 2016, the Company incurred expenses of $0.6 million, $13.5 million and $2.1 million respectively, to UCSB under the provisions of the UCSB Agreement. As of December 31, 2018 and 2017, the related accrued amounts included in the current liabilities were $3.2 million and $2.6 million, respectively. These balances are estimates based on the latest discussion with UCSB on proposed revision to the terms of the sublicense fee calculation.

Royalty obligations

The Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights. The royalty obligations are cancellable any time by giving notice to the licensor, with the termination being effective 60 days after giving notice.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Operating Lease

On December 10, 2015, the Company entered into a lease (the “Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,000 rentable square feet of office and laboratory space located in South San Francisco, California for the Company’s new corporate headquarters.

The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.

The Lease provides for annual base rent of approximately $3.1 million in the first year of the lease term. The annual base rent for the second twelve months will be approximately $4.3 million, which will increase on an annual basis beginning from the 25^th month to approximately $5.5 million for the tenth year of the lease. The Company utilized the full amount of the one-time improvement allowance of $12.6 million, of which $2.3 million is recoverable by the landlord through an increase rent which continues through the expiration of the initial lease term.

In addition, the Company obtained a standby letter of credit (the “Letter of Credit”) in an amount of approximately $0.9 million, which may be drawn by the Landlord to be applied for certain purposes upon the Company’s breach of any provisions under the 2016 Lease. The Company has recorded the $0.9 million Letter of Credit in restricted cash as non-current on its balance sheet at December 31, 2018 and 2017.

Rent expense is recognized on a straight-line basis over the term of the lease and accordingly the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability.

The future minimum lease payments for all of the Company’s facility leases are as follows (in thousands):

Year Ending December 31:
2019	$	4,854
2020		4,990
2021		5,129
2022		5,273
2023 and beyond		21,109
Total	$	41,355

Rent expense during the years ended December 31, 2018, 2017 and 2016 was $4.2 million, $4.2 million and $1.8 million, respectively.

Legal Proceedings

The Company is subject to claims and assessments from time to time in the ordinary course of business but is not aware of any such matters, individually or in the aggregate, that will have a material adverse effect on the Company’s financial position, results of operations or cash flows.

Indemnifications

In the ordinary course of business, the Company enters into agreements that may include indemnification provisions.

Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company currently has directors’ and officers’ insurance.

Common Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Common Stock

12. Common Stock

In October 2015, the Company’s board of directors and stockholders approved the amended and restatement of the Company’s certificate of incorporation. The Amended and Restated Certificate of Incorporation was effective as of October 14, 2015, which provides for 75,000,000 authorized shares of common stock with par value of $0.00001 per share and 10,000,000 shares of preferred stock with a par value of $0.00001 per share.

Common stockholders are entitled to dividends if and when declared by the Board of Directors subject to the prior rights of the preferred stockholders. As of December 31, 2018 and 2017, no dividends on common stock had been declared by the Board of Directors.

The Company had reserved shares of common stock for issuance, as follows:

	December 31,
	2018		2017
Options issued and outstanding		7,803,773		6,503,458
Shares available for future stock option grants		1,884,494		2,324,793
Shares available for future employee stock purchase plan		1,301,254		980,389
Total		10,989,521		9,808,640

Stock-based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-based Compensation

13. Stock-based Compensation

Equity Incentive Plan

In 2010, the Company adopted its 2010 Stock Incentive Plan (the “2010 Plan”) which provided for the granting of stock options to employees, directors and consultants of the Company. Options granted under the 2010 Plan were either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”).

In February 2012, the Company adopted its 2011 Stock Incentive Plan (the “2011 Plan”). The 2011 Plan is divided into two separate equity programs, an option and stock appreciation rights grant program and a stock award program. In conjunction with adopting the 2011 Plan, the Company discontinued the 2010 Plan and released the shares reserved and still available under that plan.

In connection with the consummation of the IPO in October 2015, the board of directors adopted the Company’s 2015 Equity Incentive Plan (the “2015 Plan” and collectively with the 2010 Plan and 2011 Plan, the “Plans”). In conjunction with adopting the 2015 Plan, the Company discontinued the 2011 Plan with respect to new equity awards.

The initial number of shares of common stock available for future issuance under the 2015 Plan was 2,444,735. Beginning on January 1, 2016 and continuing until the expiration of the 2015 Plan, the total number of shares of common stock available for issuance under the 2015 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of January 1 of the same year. As of December 31, 2018, 1,884,494 shares of common stock were available for future issuance under the 2015 Plan.

Stock Options

Options under the 2015 Plan may be granted for periods of up to ten years. All options issued to date have had a 10-year life.

Under the terms of the 2015 Plan, options may be granted at an exercise price not less than the estimated fair value of the shares on the date of grant, as determined by the Company’s board of directors. For employees holding more than 10% of the voting rights of all classes of stock, the exercise price of ISOs and NSOs may not be less than 110% of the estimated fair value of the shares on the date of grant, as determined by the board of directors. To date, options granted generally vest over four years and vest at a rate of 25% upon the first anniversary of the issuance date and 1/48th per month thereafter.

Activity under the Company’s stock option plans is set forth below:

							Options Outstanding
	Options Available for Grant			Number of Options			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
											(in thousands)
Balances at December 31, 2015		2,401,406			5,270,751			1.197
Options authorized		1,441,328			—		—
Options granted		(1,367,546	)		1,356,546			13.234
Options exercised		—			(414,396	)		1.549
Options forfeited		18,000			(54,155	)		4.578
Balances at December 31, 2016		2,493,188			6,158,746			3.694
Options authorized		1,459,606			—		—
Options granted		(2,138,620	)		2,138,620			13.566
Options exercised		—			(764,576	)		4.140
Options forfeited		510,619			(1,029,332	)		9.118
Balances at December 31, 2017		2,324,793			6,503,458			8.157
Options authorized		1,539,142						—
Options granted		(2,127,400	)		2,127,400			24.654
Options exercised		—			(673,382	)		6.172
Options forfeited		147,959			(153,703	)		14.293
Balances at December 31, 2018		1,884,494			7,803,773			12.622		7.3	$	40,901
Options Exercisable—December 31, 2018					4,520,806			7.853		6.3	$	36,017
Options vested and expected to vest—December 31, 2018					7,803,773			12.622		7.3	$	40,901

Information with respect to stock options outstanding and exercisable as of December 31, 2018 is as follows:

	Options Outstanding				Options Exercisable
Range of Exercise Price	Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable		Weighted- Average Exercise Price
$0.945 - $1.386		820,235		3.4		820,235	$	1.174
$1.449 - $1.575		920,347		5.9		915,120	$	1.545
$4.473 - $4.473		119,531		6.4		119,531	$	4.473
$6.615 - $6.615		1,171,404		6.6		910,002	$	6.615
$10.170 - $11.640		497,541		7.7		298,494	$	10.715
$11.940 - $11.940		842,500		8.1		403,695	$	11.940
$12.000 - $14.460		1,073,581		7.5		648,933	$	14.172
$14.620 - $25.670		846,434		9.1		157,099	$	18.574
$25.820 - $25.820		885,000		9.1		202,809	$	25.820
$26.300 - $29.710		627,200		9.0		44,888	$	26.740
		7,803,773		7.3		4,520,806	$	7.853

The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the estimated fair value of the underlying common stock as of December 31, 2018, 2017 and 2016, respectively.

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2018, 2017 and 2016 was $14.6 million, $10.5 million and $4.6 million, respectively.

The options granted in the years ended December 31, 2018, 2017 and 2016 had a weighted-average per share grant-date fair value of $14.205, $8.207, and $8.936, respectively. At December 31, 2018, the unrecognized compensation expense with respect to options granted to employees was $34.6 million, and is expected to be recognized over 2.4 years.

Early Exercise of Employee Options

Certain stock options granted under the Plans provide option holders the right to elect to exercise unvested options in exchange for restricted common stock. Such unvested restricted shares are subject to a repurchase right held by the Company at the original issuance price in the event the optionee’s service to the Company is terminated either voluntarily or involuntarily. The right usually lapses 25% on the first anniversary of the vesting start date and in 36 equal monthly amounts thereafter. These repurchase terms are considered to be a forfeiture provision. The cash or full recourse notes received from employees for exercise of unvested options is treated as a refundable deposit and is classified as a liability on the balance sheets.

Employee Stock Purchase Plan

Concurrent with the completion of the IPO in October 2015, the Company’s Employee Stock Purchase Plan (“ESPP”) became effective. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for six-month offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company issued 63,920 shares and 67,746 shares of common stock under the ESPP in 2018 and 2017, respectively.

Shares available for future purchase under the ESPP were 1,301,254 at December 31, 2018. The compensation expense related to the ESPP was $0.5 million, $0.2 million and $0.1 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018 and 2017, there was $0.3 million and $0.1 million, respectively, of unrecognized compensation cost related to the ESPP, which the Company expects to recognize over 5 months.

Stock Based Compensation

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	8,313	$	5,161	$	4,925
General and administrative		8,565		6,126		5,170
Total stock-based compensation expense	$	16,878	$	11,287	$	10,095

Stock-based compensation expense for employees was $16.7 million, $11.0 million and $9.4 million for the years ended December 31, 2018, 2017 and 2016, respectively.

Stock-based compensation expense related to stock options granted to non-employees is recognized as the stock options are earned. The Company determined that the estimated fair value of the stock options is more readily measurable than the fair value of the services received. The fair value of stock options granted to non-employees is calculated at each grant date and re-measured at each reporting date using the Black-Scholes option pricing model. The stock-based compensation expense related to a grant will fluctuate as the estimated fair value of the common stock fluctuates over the period from the grant date to the vesting date.

Stock-based compensation expense for non-employees was $0.2 million, $0.3 million and $0.9 million for the years ended December 31, 2018, 2017 and 2016, respectively.

The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions:

	Options			ESPP
	Year Ended December 31,			Year Ended December 31,
	2018	2017	2016	2018	2017	2016
Expected volatility	65.6%-69.3%	69.1%-72.4%	76.4% – 83.5%	46.4%-70.5%	52.3%-63.8%	50.4% – 75.6%
Risk-free interest rate	2.5%-3.0%	1.7%-2.2%	1.2% – 2.1%	2.1%-2.6%	1.1%-1.5%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %	— %	— %
Expected term (in years)	4.7-4.9	4.9-5.3	5.3 – 5.9	0.5	0.5	0.5

Expected Term. The expected term of stock options represents the period that the stock options are expected to remain outstanding and is based on vesting terms, exercise term and contractual lives of the options. The expected term of the ESPP shares is equal to the six-month look-back period.

Expected Volatility. The expected stock price volatility for the Company’s stock options was derived from the average historical volatilities of the Company’s stock price and the stock price of several comparable publicly traded companies within the biotechnology and pharmaceutical industry. The Company will continue to apply this process until a sufficient amount of historical information on the Company’s own stock price becomes available. Volatility for ESPP shares is equal to our historical volatility over the six-month look-back period.

Risk-Free Interest Rate. The risk-free interest rate is based on the U.S. Treasury whose term was consistent with expected term of the Company’s stock options.

Dividend Rate. The expected dividend was assumed to be zero as the Company has never paid dividends and has no current plans to do so.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

14. Income Taxes

The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands):

	Years Ended December 31,
	2018		2017			2016
Current:
Federal	$	14,302	$	—		$	—
State		1		1			1
Total current		14,303		1			1
Deferred:
Federal		—		(514	)		(20	)
State		—		—			—
Total deferred		—		(514	)		(20	)
Provision for (benefit from) income taxes	$	14,303	$	(513	)	$	(19	)

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	Years Ended December 31,
	2018			2017			2016
U.S. federal taxes at statutory rate		21.0	%		34.0	%		34.0	%
State tax, net of federal benefit		0.7	%		7.6	%		0.8	%
Stock compensation		1.7	%		2.3	%		(0.6	)%
Tax attributes subject to 382 limitation		0.0	%		27.5	%		0.0	%
Tax credits		6.2	%		2.7	%		2.2	%
Change in valuation allowance		(49.5	)%		(9.3	)%		(35.6	)%
Change in deferreds due to rate change		0.0	%		(58.6	)%		0.0	%
Other		(0.5	)%		(5.0	)%		(0.8	)%
Total		(20.4	)%		1.2	%		0.0	%

The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands):

	Year Ended December 31,
	2018			2017			2016
Net operating loss carryforwards	$	15,468		$	24,682		$	24,528
Research and development credits		7,514			5,757			2,683
Intangible—in-process R&D		—			—			81
Deferred revenue		56,881			15,631			13,857
Accruals and deferred rent		1,955			1,256			1,335
Stock-based compensation		5,746			3,831			3,963
Other		39			32			1
Total gross deferred income tax assets		87,603			51,189			46,448
Less: valuation allowance		(86,466	)		(50,791	)		(46,137	)
Deferred tax assets, net of valuation allowance		1,137			398			311
Fixed assets		(854	)		(282	)		(229	)
In-process R&D		—			—			(595	)
Intangible assets		(108	)		(116	)		—
Prepaid Expenses		(175	)		—			—
Deferred tax liabilities		(1,137	)		(398	)		(824	)
Net deferred income tax liabilities	$	—		$	—		$	(513	)

The Company has established a valuation allowance against all of its net deferred tax assets. Management considered all available evidence, both positive and negative, including but not limited to our historical operating results, income or loss in recent periods, cumulative losses in recent years, forecasted earnings, future taxable income, and significant risk and uncertainty related to forecasts, and concluded the deferred tax assets are not more likely than not to be realized. The net change in the total valuation allowance for the years ended December 31, 2018, 2017 and 2016 was an increase of $35.7 million, $4.7 million and $21.1 million, respectively.

The Company had net operating loss carryforwards for federal and state income tax purposes of approximately $65.6 million and $24.2 million, respectively, as of December 31, 2018, available to reduce future taxable income. The federal and state net operating loss carryforwards will begin to expire in 2034 and 2033, respectively, if not utilized.

The Company also has federal and state research and development tax credits carryforwards of $5.7 million and $5.5 million, respectively, as of December 31, 2018 available to reduce future income taxes. The federal research and development tax credits will begin to expire in 2031 if not utilized. The state research and development tax credits have no expiration date.

In December 2017, the Tax Cuts and Jobs Act (“Tax Act”) was signed into law making significant changes to the Internal Revenue Code. Changes include, but are not limited to, a corporate tax rate decrease from 35% to 21% effective for tax years beginning after December 31, 2017 the transition of U.S international taxation from a worldwide tax system to a territorial system, and a one-time transition tax on the mandatory deemed repatriation of cumulative foreign earnings. Under the Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”) issued by the SEC staff in December 2017, it allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company had calculated its best estimate of the impact of the Tax Act in accordance with its understanding of the Tax Act and guidance available at the time. The tax rate decrease resulted in a reduction of $25.7 million in its deferred tax assets, and a corresponding decrease of the same amount in the valuation allowance against these deferred tax assets as at December 31, 2017, as substantially all of its U.S. deferred tax assets, net of deferred tax liabilities, are subject to a full valuation allowance. The deferred tax asset remeasurement as at December 31, 2017 was provisional because the Company continued to evaluate the impact of various domestic provisions of the Act as well as the impact of additional guidance that may be provided. In the fourth quarter of 2018, the Company completed its related analysis to determine the effect of the Tax Act. No material adjustments were recorded based on completion of the related analysis as of December 31, 2018

In 2018, the Company adopted ASC 606 under the modified retrospective transition method and recorded a net transition adjustment of $10.9 million to accumulated deficit associated with the change in timing of income recognition for the Company’s collaboration agreements. The transition adjustment also resulted in a change to the deferred revenue related deferred tax balance, offset by an adjustment to the valuation allowance.

Internal Revenue Code section 382 (“IRC Section 382”) places a limitation (the “Section 382 Limitation”) on the amount of taxable income that can be offset by net operating loss (“NOL”) carryforwards after a change in control (generally greater than 50% change in ownership) of a loss corporation. California has similar rules. The Company has performed an IRC Section 382 analysis and determined there was an ownership change in 2017 that resulted in 382 limitations. When an ownership change occurs, IRC Section 382 limits the use of NOLs and credits in subsequent periods based on the annual 382 limitations. The annual 382 limitations may limit the full use of available tax attributes in one year but the identified ownership changes may not result in expiration of tax attributes for use prior to expiration of their respective carryforward periods. Accordingly, none of the tax attributes have been reduced but limited the full use in 2018. There may be further ownership changes after December 31, 2017. The Company has determined that, while an ownership change has occurred, the applicable limits would not impair the value or anticipated use of the Company’s federal and state net operating losses. Although realization is not assured, management believes it is more likely than not that any limitation under IRC Section 382 will not impair the realizability of the deferred income tax assets related to federal and state net operating loss carryforwards.

The Company had approximately $3.8 million and $4.3 million of unrecognized tax benefits as of December 31, 2018 and 2017, respectively, and approximately $1.0 million and $0, respectively, would affect the Company’s effective tax rate if recognized.

A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands):

	Year Ended December 31,
	2018			2017		2016
Balance at the beginning of the year	$	4,320		$	1,182	$	666
Additions based on tax positions related to current year		1,166			521		23
Adjustment based on tax positions related to prior years		(1,730	)		2,617		493
Balance at end of the year	$	3,756		$	4,320	$	1,182

The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. To the extent accrued interest and penalties do not ultimately become payable, amounts accrued will be reduced and reflected as a reduction of the provision for income taxes in the period that such determination is made. Interest and penalties have not been accrued for 2018, 2017 and 2016.

The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2018 remains open to U.S. federal and state examination to the extent of the utilization of net operating loss and credit carryovers. As of December 31, 2018, the Company is not under examination by the Internal Revenue Service or any state or foreign tax jurisdiction.

Related Party Transactions

12 Months Ended

Dec. 31, 2018

Related Party Transactions Disclosure

15. Related Party Transactions

One of the Company’s board members was affiliated with Third Rock Ventures, a greater than 10% stockholder of the Company in 2017. General and administrative expenses for board service fees incurred were $35,000 and $48,000 for the years ended December 31, 2017 and 2016, respectively. The amounts outstanding and included in accounts payable were $0 and $12,000 as of December 31, 2017 and December 31, 2016, respectively. This board member resigned in December 2017.

Revenues from related parties refer to the collaboration agreement with Pfizer, one of the Company’s stockholders. The Company recognized revenue of $2.2 million for the year ended December 31, 2016. As of December 31, 2016, deferred revenue relating to the Pfizer Agreement was $3.4 million. The amount due from Pfizer under the agreement was $0.1 million as of December 31, 2016. As of and during the year ended December 31, 2017, Pfizer owned less than 10% of the Company’s outstanding common stock and was no longer a related party for purposes of disclosure.

Supplementary Data - Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Supplementary Data – Quarterly Financial Data (Unaudited)

17. Supplementary Data – Quarterly Financial Data (Unaudited)

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2018 and 2017:

	Three Months Ended
(in thousands, except per share data)	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
Revenue	$	11,471		$	12,509		$	21,338		$	14,184
Net income (loss)	$	(32,233	)	$	(23,431	)	$	(13,447	)	$	(15,493	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.72	)	$	(0.53	)	$	(0.35	)	$	(0.40	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2017		September 30, 2017			June 30, 2017			March 31, 2017
Revenue	$	27,074	$	24,144		$	8,752		$	11,653
Net income (loss)	$	621	$	(10,247	)	$	(25,216	)	$	(8,257	)
Net loss per share attributable to common stockholders, basic and diluted	$	0.02	$	(0.28	)	$	(0.69	)	$	(0.23	)

Basis of Presentation and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“U.S. GAAP”).

Use of Estimates

Concentration of Credit Risk and Other Risks and Uncertainties

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2018		2017		2016		2018		2017
AbbVie Ireland Unlimited Company	$	18,997	$	19,434	$	3,268		—	**
Bristol-Myers Squibb Company		32,780		36,492		9,577		97		10,126
ImmunoGen, Inc.	*			12,503		—		—		—
Pfizer Inc.	*		*			2,198		—	**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	51,777	$	68,429	$	15,043	$	97	$	10,126

*	Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented.

**	Accounts receivable balance from the customer was less than 10% of the Company’s total accounts receivable as of the respective periods presented.

All of the Company’s customers are located in the United States of America.

Segments

Cash and Cash Equivalents

The Company considers all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents.

Restricted Cash

Restricted cash represents a standby letter of credit issued pursuant to an office lease entered in December 2015.

	December 31
	2018		2017
Cash and cash equivalents	$	247,577	$	177,548
Restricted cash		917		917
	$	248,494	$	178,465

Short-term Investments

Property and Equipment, net

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Maintenance and repairs that do not extend the life or improve the asset are expensed when incurred.

Goodwill and Intangible Assets

Impairment of Long-Lived Assets

Revenue Recognition

Comprehensive Loss

Contract Balances

Stock-Based Compensation

Income Taxes

Accrued Research and Development Costs

Reclassifications

Adopted Accounting Pronouncements and Recent Accounting Pronouncements

Adopted Accounting Pronouncements

The following table summarizes the impact of adopting ASC 606 on select balance sheet line items (in thousands):

	As of December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Liabilities
Income tax payable	$	14,886		$	(1,547	)	$	13,339
Other long-term liabilities		3,249			(199	)		3,050
Deferred revenue - current		45,288			7,425			52,713
Deferred revenue - long-term		214,790			10,477			225,267
Stockholders' Equity
Accumulated deficit		(298,825	)		(16,156	)		(314,981	)

	Three Months Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	10,022		$	1,449		$	11,471
Loss from Operations		(27,260	)		1,449			(25,811	)
Loss before income taxes		(24,770	)		1,449			(23,321	)
Provision for income taxes		11,566			(2,654	)		8,912
Net loss		(36,336	)		4,103			(32,233	)
Net loss per share, basic and diluted		(0.81	)		0.09			(0.72	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	65,966		$	(6,464	)	$	59,502
Loss from Operations		(71,410	)		(6,464	)		(77,874	)
Loss before income taxes		(63,837	)		(6,464	)		(70,301	)
Provision for income taxes		16,049			(1,746	)		14,303
Net loss		(79,886	)		(4,718	)		(84,604	)
Net loss per share, basic and diluted		(1.92	)		(0.11	)		(2.03	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Cash flows from operating activities:
Net loss	$	(79,886	)	$	(4,718	)	$	(84,604	)
Changes in operating assets and liabilities:
Accrued liabilities, income tax payable and other long-term liabilities		26,579			(1,746	)		24,833
Deferred revenue		(44,659	)		6,464			(38,195	)

Recent Accounting Pronouncements

Basis of Presentation and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance

	Revenue						Accounts Receivable, net
	Year Ended December 31,						December 31,
	2018		2017		2016		2018		2017
AbbVie Ireland Unlimited Company	$	18,997	$	19,434	$	3,268		—	**
Bristol-Myers Squibb Company		32,780		36,492		9,577		97		10,126
ImmunoGen, Inc.	*			12,503		—		—		—
Pfizer Inc.	*		*			2,198		—	**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	51,777	$	68,429	$	15,043	$	97	$	10,126

*	Revenue from the customer was less than 10% of the Company’s total revenue for the respective periods presented.

**	Accounts receivable balance from the customer was less than 10% of the Company’s total accounts receivable as of the respective periods presented.

Schedule of Cash, Cash Equivalents, and Restricted Cash

	December 31
	2018		2017
Cash and cash equivalents	$	247,577	$	177,548
Restricted cash		917		917
	$	248,494	$	178,465

Schedule of Useful Lives of Property and Equipment

The useful lives of property and equipment are as follows:

Machinery and equipment		5 years
Computer equipment and software		3 years
Furniture and fixtures		3 years
Leasehold improvements		Shorter of remaining lease term or estimated life of the assets

Adopted and Recent Accounting Pronouncements (Tables)

12 Months Ended

Dec. 31, 2018

ASC 606

Summary Impact of Adopting ASC 606 on Select Financial Statement Line Items

The following table summarizes the impact of adopting ASC 606 on select balance sheet line items (in thousands):

	As of December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Liabilities
Income tax payable	$	14,886		$	(1,547	)	$	13,339
Other long-term liabilities		3,249			(199	)		3,050
Deferred revenue - current		45,288			7,425			52,713
Deferred revenue - long-term		214,790			10,477			225,267
Stockholders' Equity
Accumulated deficit		(298,825	)		(16,156	)		(314,981	)

	Three Months Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	10,022		$	1,449		$	11,471
Loss from Operations		(27,260	)		1,449			(25,811	)
Loss before income taxes		(24,770	)		1,449			(23,321	)
Provision for income taxes		11,566			(2,654	)		8,912
Net loss		(36,336	)		4,103			(32,233	)
Net loss per share, basic and diluted		(0.81	)		0.09			(0.72	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Revenue	$	65,966		$	(6,464	)	$	59,502
Loss from Operations		(71,410	)		(6,464	)		(77,874	)
Loss before income taxes		(63,837	)		(6,464	)		(70,301	)
Provision for income taxes		16,049			(1,746	)		14,303
Net loss		(79,886	)		(4,718	)		(84,604	)
Net loss per share, basic and diluted		(1.92	)		(0.11	)		(2.03	)

	Year Ended December 31, 2018
	Balances Under ASC 605			Adjustments			As Reported Under ASC 606
Cash flows from operating activities:
Net loss	$	(79,886	)	$	(4,718	)	$	(84,604	)
Changes in operating assets and liabilities:
Accrued liabilities, income tax payable and other long-term liabilities		26,579			(1,746	)		24,833
Deferred revenue		(44,659	)		6,464			(38,195	)

Net Loss Per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share

The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share as their effect would have been anti-dilutive:

	Year Ended December 31,
	2018		2017		2016
Options to purchase common stock		7,478,755		6,891,123		6,086,939

Fair Value Measurements and Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Financial Instruments Subject to Fair Value Measurements on a Recurring Basis

		December 31, 2018
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	226,979	$	—	$	—		$	226,979
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		188,616		—		(66	)		188,550
Total Securities		$	416,512	$	—	$	(66	)	$	416,446

		December 31, 2017
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	164,440	$	—	$	—		$	164,440
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level I		196,629		—		(67	)		196,562
Total Securities		$	361,986	$	—	$	(67	)	$	361,919

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following (in thousands):

	December 31
	2018			2017
Machinery and equipment	$	10,498		$	7,162
Computer equipment and software		955			897
Furniture and fixtures		749			643
Leasehold improvements		893			701
Construction in progress		785			23
		13,880			9,426
Less: accumulated depreciation and amortization		(6,946	)		(5,208	)
	$	6,934		$	4,218

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2018

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

Goodwill and intangible assets consisted of the following (in thousands):

Goodwill
	December 31,
	2018			2017
Goodwill	$	949		$	949

Intangible assets
	December 31,
	2018			2017
In-process research and development	$	1,750		$	1,750
Less accumulated amortization		(292	)		(146	)
	$	1,458		$	1,604

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2018		2017
Research and clinical expenses	$	18,520	$	10,068
Payroll and related expenses		6,585		4,526
Legal and professional expenses		830		1,523
Other accrued expenses		789		265
Total	$	26,724	$	16,382

Research and Collaboration Agreements (Tables)

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Schedule of Revenue by Collaboration Partner

The following table summarizes the revenue by collaboration partner (in thousands):

	Year Ended December 31,
	2018		2017		2016
AbbVie	$	18,997	$	19,434	$	3,268
Amgen		4,899		1,311		—
BMS		32,780		36,492		9,577
ImmunoGen		1,471		12,503		—
Pfizer		1,355		1,883		2,198
Total Revenue	$	59,502	$	71,623	$	15,043

Summary of Contract Liabilities

The following table presents changes in the Company’s total contract liabilities for the year ended December 31, 2018 (in thousands):

			Additions				Deductions
	Balance at 12/31/2017		ASC 606 Adoption Adjustment		Adjustment to Transaction Price from Performance Obligation Satisfied		Revenue Recognized from an Adjustment to Transaction Price During the Period			Revenue Recognized from Amounts Included in Contract Liability at the Beginning of the Period			Balance at 12/31/2018
Contract liabilities:
Deferred revenue	$	305,263	$	10,912	$	21,000	$	(11,718	)	$	(47,477	)	$	277,980

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Future Minimum Lease Payments

The future minimum lease payments for all of the Company’s facility leases are as follows (in thousands):

Year Ending December 31:
2019	$	4,854
2020		4,990
2021		5,129
2022		5,273
2023 and beyond		21,109
Total	$	41,355

Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Schedule Reserved Shares of Common Stock Issuance on Converted Basis

The Company had reserved shares of common stock for issuance, as follows:

	December 31,
	2018		2017
Options issued and outstanding		7,803,773		6,503,458
Shares available for future stock option grants		1,884,494		2,324,793
Shares available for future employee stock purchase plan		1,301,254		980,389
Total		10,989,521		9,808,640

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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-Topic 210
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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+ Details

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Activity Under Company's Stock Option Plans

Activity under the Company’s stock option plans is set forth below:

							Options Outstanding
	Options Available for Grant			Number of Options			Weighted- Average Exercise Price Per Share		Weighted- Average Remaining Contractual Life (years)		Aggregate Intrinsic Value
											(in thousands)
Balances at December 31, 2015		2,401,406			5,270,751			1.197
Options authorized		1,441,328			—		—
Options granted		(1,367,546	)		1,356,546			13.234
Options exercised		—			(414,396	)		1.549
Options forfeited		18,000			(54,155	)		4.578
Balances at December 31, 2016		2,493,188			6,158,746			3.694
Options authorized		1,459,606			—		—
Options granted		(2,138,620	)		2,138,620			13.566
Options exercised		—			(764,576	)		4.140
Options forfeited		510,619			(1,029,332	)		9.118
Balances at December 31, 2017		2,324,793			6,503,458			8.157
Options authorized		1,539,142						—
Options granted		(2,127,400	)		2,127,400			24.654
Options exercised		—			(673,382	)		6.172
Options forfeited		147,959			(153,703	)		14.293
Balances at December 31, 2018		1,884,494			7,803,773			12.622		7.3	$	40,901
Options Exercisable—December 31, 2018					4,520,806			7.853		6.3	$	36,017
Options vested and expected to vest—December 31, 2018					7,803,773			12.622		7.3	$	40,901

Schedule of Stock Option Outstanding and Exercisable

Information with respect to stock options outstanding and exercisable as of December 31, 2018 is as follows:

	Options Outstanding				Options Exercisable
Range of Exercise Price	Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable		Weighted- Average Exercise Price
$0.945 - $1.386		820,235		3.4		820,235	$	1.174
$1.449 - $1.575		920,347		5.9		915,120	$	1.545
$4.473 - $4.473		119,531		6.4		119,531	$	4.473
$6.615 - $6.615		1,171,404		6.6		910,002	$	6.615
$10.170 - $11.640		497,541		7.7		298,494	$	10.715
$11.940 - $11.940		842,500		8.1		403,695	$	11.940
$12.000 - $14.460		1,073,581		7.5		648,933	$	14.172
$14.620 - $25.670		846,434		9.1		157,099	$	18.574
$25.820 - $25.820		885,000		9.1		202,809	$	25.820
$26.300 - $29.710		627,200		9.0		44,888	$	26.740
		7,803,773		7.3		4,520,806	$	7.853

Total Stock-based Compensation Recognized

Total stock-based compensation recorded related to options granted to employees and non-employees and employee stock purchase plan was as follows (in thousands):

	Year Ended December 31,
	2018		2017		2016
Research and development	$	8,313	$	5,161	$	4,925
General and administrative		8,565		6,126		5,170
Total stock-based compensation expense	$	16,878	$	11,287	$	10,095

Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model

The Company estimated the fair value of employee stock options and ESPP using the Black-Scholes valuation model based on the date of grant with the following assumptions:

	Options			ESPP
	Year Ended December 31,			Year Ended December 31,
	2018	2017	2016	2018	2017	2016
Expected volatility	65.6%-69.3%	69.1%-72.4%	76.4% – 83.5%	46.4%-70.5%	52.3%-63.8%	50.4% – 75.6%
Risk-free interest rate	2.5%-3.0%	1.7%-2.2%	1.2% – 2.1%	2.1%-2.6%	1.1%-1.5%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %	— %	— %
Expected term (in years)	4.7-4.9	4.9-5.3	5.3 – 5.9	0.5	0.5	0.5

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Components of Provision for (Benefit from) Income Taxes

The Company derives its income only from the United States. The components of the provision for (benefit from) income taxes are as follows (in thousands):

	Years Ended December 31,
	2018		2017			2016
Current:
Federal	$	14,302	$	—		$	—
State		1		1			1
Total current		14,303		1			1
Deferred:
Federal		—		(514	)		(20	)
State		—		—			—
Total deferred		—		(514	)		(20	)
Provision for (benefit from) income taxes	$	14,303	$	(513	)	$	(19	)

Schedule of Effective Tax Rate Reconciliation

A reconciliation of the Company’s effective tax rate to the statutory U.S. federal rate is as follows:

	Years Ended December 31,
	2018			2017			2016
U.S. federal taxes at statutory rate		21.0	%		34.0	%		34.0	%
State tax, net of federal benefit		0.7	%		7.6	%		0.8	%
Stock compensation		1.7	%		2.3	%		(0.6	)%
Tax attributes subject to 382 limitation		0.0	%		27.5	%		0.0	%
Tax credits		6.2	%		2.7	%		2.2	%
Change in valuation allowance		(49.5	)%		(9.3	)%		(35.6	)%
Change in deferreds due to rate change		0.0	%		(58.6	)%		0.0	%
Other		(0.5	)%		(5.0	)%		(0.8	)%
Total		(20.4	)%		1.2	%		0.0	%

Schedule of Deferred Income Tax Liabilities

The types of temporary differences that give rise to significant portions of the Company’s deferred income tax liabilities are set out below (in thousands):

	Year Ended December 31,
	2018			2017			2016
Net operating loss carryforwards	$	15,468		$	24,682		$	24,528
Research and development credits		7,514			5,757			2,683
Intangible—in-process R&D		—			—			81
Deferred revenue		56,881			15,631			13,857
Accruals and deferred rent		1,955			1,256			1,335
Stock-based compensation		5,746			3,831			3,963
Other		39			32			1
Total gross deferred income tax assets		87,603			51,189			46,448
Less: valuation allowance		(86,466	)		(50,791	)		(46,137	)
Deferred tax assets, net of valuation allowance		1,137			398			311
Fixed assets		(854	)		(282	)		(229	)
In-process R&D		—			—			(595	)
Intangible assets		(108	)		(116	)		—
Prepaid Expenses		(175	)		—			—
Deferred tax liabilities		(1,137	)		(398	)		(824	)
Net deferred income tax liabilities	$	—		$	—		$	(513	)

Schedule of Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending unrecognized tax benefit amount is as follows (in thousands):

	Year Ended December 31,
	2018			2017		2016
Balance at the beginning of the year	$	4,320		$	1,182	$	666
Additions based on tax positions related to current year		1,166			521		23
Adjustment based on tax positions related to prior years		(1,730	)		2,617		493
Balance at end of the year	$	3,756		$	4,320	$	1,182

Supplementary Data - Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Unaudited Financial Information

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2018 and 2017:

	Three Months Ended
(in thousands, except per share data)	December 31, 2018			September 30, 2018			June 30, 2018			March 31, 2018
Revenue	$	11,471		$	12,509		$	21,338		$	14,184
Net income (loss)	$	(32,233	)	$	(23,431	)	$	(13,447	)	$	(15,493	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.72	)	$	(0.53	)	$	(0.35	)	$	(0.40	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2017		September 30, 2017			June 30, 2017			March 31, 2017
Revenue	$	27,074	$	24,144		$	8,752		$	11,653
Net income (loss)	$	621	$	(10,247	)	$	(25,216	)	$	(8,257	)
Net loss per share attributable to common stockholders, basic and diluted	$	0.02	$	(0.28	)	$	(0.69	)	$	(0.23	)

Description of the Business - Additional Information (Detail) - Common Stock - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Jul. 31, 2018	Oct. 04, 2017	Dec. 31, 2018
Description Of Business [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares			5,867,347
Net proceeds from sale of common stock	$ 134.6
Underwriting discounts and commissions and offering expenses	$ 9.2
Amgen Inc
Description Of Business [Line Items]
Common stock, aggregate cash purchase price		$ 20.0
Underwritten Public Offering
Description Of Business [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	5,867,347
Common stock price, per share	$ 24.50
Underwriters
Description Of Business [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares	765,306
Private Placement \| Amgen Inc
Description Of Business [Line Items]
Issuance of common stock in follow-on offering, net of issuance costs, shares		1,156,069
Common stock price, per share		$ 17.30
Period used to calculate weighted average price per share		20 days

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Customers and Collaboration Partners who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Revenue	$ 11,471	$ 12,509	$ 21,338	$ 14,184	$ 27,074	$ 24,144	$ 8,752	$ 11,653	$ 59,502	$ 71,623	$ 12,845
Accounts Receivable, net	97				10,139				97	10,139
AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue									19,000	19,400	3,300
Major Customer
Concentration Risk [Line Items]
Revenue									51,777	68,429	15,043
Accounts Receivable, net	97				10,126				97	10,126
Major Customer \| AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue									18,997	19,434	3,268
Major Customer \| Bristol Myers Squibb Company
Concentration Risk [Line Items]
Revenue									32,780	36,492	9,577
Accounts Receivable, net	$ 97				$ 10,126				$ 97	10,126
Major Customer \| Immuno Gen Inc
Concentration Risk [Line Items]
Revenue										$ 12,503
Major Customer \| Pfizer Inc
Concentration Risk [Line Items]
Revenue											$ 2,198

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Activity Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounting Policies [Abstract]
Number of business activities \| Activity	1
Number of operating segments \| Segment	1
Impairment of goodwill	$ 0	$ 0	$ 0
Impairment of intangible assets	0	0	0
Impairment of long-lived assets	$ 0	$ 0	$ 0

Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 247,577	$ 177,548
Restricted cash	917	917
Cash, cash equivalents, and restricted cash	$ 248,494	$ 178,465	$ 105,562	$ 60,739

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Machinery and equipment
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	5 years
Computer equipment and software
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Furniture and fixtures
Property Plant And Equipment [Line Items]
Estimated useful life of property and equipment	3 years
Leasehold improvements
Property Plant And Equipment [Line Items]
Estimated useful life of leasehold improvements, description	Shorter of remaining lease term or estimated life of the assets

Adopted and Recent Accounting Pronouncements - Additional Information (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Increase in accumulated deficit balance		$ (314,981)		$ (219,465)
ASC 842 \| Subsequent Event \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset and lease liability	$ 27,000
ASC 842 \| Subsequent Event \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset and lease liability	$ 32,000
Adjustments \| ASC 606
Summary Of Significant Accounting Policies [Line Items]
Increase in accumulated deficit balance		$ (16,156)	$ (10,900)

Adopted and Recent Accounting Pronouncements - Summary Impact of Adopting ASC 606 on Select Condensed Balance Sheet Line Items (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Liabilities
Income tax payable	$ 13,339		$ 1
Deferred revenue - current	52,713		40,559
Deferred revenue - long-term	225,267		264,704
Other long-term liabilities	3,050		1,897
Stockholders' Equity
Accumulated deficit	(314,981)		$ (219,465)
ASC 606 \| Balances Under ASC 605
Liabilities
Income tax payable	14,886
Deferred revenue - current	45,288
Deferred revenue - long-term	214,790
Other long-term liabilities	3,249
Stockholders' Equity
Accumulated deficit	(298,825)
ASC 606 \| Adjustments
Liabilities
Income tax payable	(1,547)
Deferred revenue - current	7,425
Deferred revenue - long-term	10,477
Other long-term liabilities	(199)
Stockholders' Equity
Accumulated deficit	$ (16,156)	$ (10,900)

Adopted and Recent Accounting Pronouncements - Summary Impact of Adopting ASC 606 on Select Condensed Statement of Operations Line Items (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	$ 3,050				$ 1,897				$ 3,050	$ 1,897
Revenue	11,471								59,502	71,623	$ 15,043
Loss from Operations	(25,811)								(77,874)	(46,259)	(59,586)
Loss before income taxes	(23,321)								(70,301)	(43,612)	(58,919)
Provision for income taxes	8,912								14,303	(513)	(19)
Net loss	$ (32,233)	$ (23,431)	$ (13,447)	$ (15,493)	$ 621	$ (10,247)	$ (25,216)	$ (8,257)	$ (84,604)	$ (43,099)	$ (58,900)
Net loss per share, basic and diluted	$ (0.72)	$ (0.53)	$ (0.35)	$ (0.40)	$ 0.02	$ (0.28)	$ (0.69)	$ (0.23)	$ (2.03)	$ (1.16)	$ (1.63)
ASC 606 \| Balances Under ASC 605
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	$ 3,249								$ 3,249
Revenue	10,022								65,966
Loss from Operations	(27,260)								(71,410)
Loss before income taxes	(24,770)								(63,837)
Provision for income taxes	11,566								16,049
Net loss	$ (36,336)								$ (79,886)
Net loss per share, basic and diluted	$ (0.81)								$ (1.92)
ASC 606 \| Adjustments
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	$ (199)								$ (199)
Revenue	1,449								(6,464)
Loss from Operations	1,449								(6,464)
Loss before income taxes	1,449								(6,464)
Provision for income taxes	(2,654)								(1,746)
Net loss	$ 4,103								$ (4,718)
Net loss per share, basic and diluted	$ 0.09								$ (0.11)

Adopted and Recent Accounting Pronouncements - Summary Impact of Adopting ASC 606 on Select Condensed Statement of Cash Flows Line Items (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	$ 3,050				$ 1,897				$ 3,050	$ 1,897
Cash flows from operating activities:
Net loss	(32,233)	$ (23,431)	$ (13,447)	$ (15,493)	$ 621	$ (10,247)	$ (25,216)	$ (8,257)	(84,604)	(43,099)	$ (58,900)
Changes in operating assets and liabilities
Accrued liabilities, income tax payable and other long-term liabilities									24,833
Deferred revenue									(38,195)	$ 189,913	$ 43,317
ASC 606 \| Balances Under ASC 605
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	3,249								3,249
Cash flows from operating activities:
Net loss	(36,336)								(79,886)
Changes in operating assets and liabilities
Accrued liabilities, income tax payable and other long-term liabilities									26,579
Deferred revenue									(44,659)
ASC 606 \| Adjustments
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Other long-term liabilities	(199)								(199)
Cash flows from operating activities:
Net loss	$ 4,103								(4,718)
Changes in operating assets and liabilities
Accrued liabilities, income tax payable and other long-term liabilities									(1,746)
Deferred revenue									$ 6,464

Fair Value Measurements and Short-Term Investments - Schedule of Financial Instruments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 416,512	$ 361,986
Gross Unrealized Holding Losses	(66)	(67)
Aggregate Fair Value	416,446	361,919
Level I \| Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	226,979	164,440
Aggregate Fair Value	226,979	164,440
Level I \| Restricted cash (money market funds)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	917	917
Aggregate Fair Value	917	917
Level I \| U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	188,616	196,629
Gross Unrealized Holding Losses	(66)	(67)
Aggregate Fair Value	$ 188,550	$ 196,562

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Abstract]
Machinery and equipment	$ 10,498	$ 7,162
Computer equipment and software	955	897
Furniture and fixtures	749	643
Leasehold improvements	893	701
Construction in progress	785	23
Total property and equipment	13,880	9,426
Less: accumulated depreciation and amortization	(6,946)	(5,208)
Property and equipment, net	$ 6,934	$ 4,218

Goodwill and Intangible Assets - Additional Information (Details) - IPR&D [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Average estimated lives of patents	12 years
Amortization of intangible assets	$ 0.1	$ 0.1

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Goodwill
Goodwill	$ 949	$ 949
In-Process Research and Development
Intangible assets
Intangible assets, gross	1,750	1,750
Less accumulated amortization	(292)	(146)
Intangible assets, net	$ 1,458	$ 1,604

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 18,520	$ 10,068
Payroll and related expenses	6,585	4,526
Legal and professional expenses	830	1,523
Other accrued expenses	789	265
Total	$ 26,724	$ 16,382

Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partner (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue	$ 11,471	$ 59,502	$ 71,623	$ 15,043
AbbVie
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		18,997	19,434	3,268
Amgen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		4,899	1,311
BMS
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		32,780	36,492	9,577
ImmunoGen
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		1,471	12,503
Pfizer
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue		$ 1,355	$ 1,883	$ 2,198

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)	1 Months Ended			3 Months Ended								12 Months Ended
	May 31, 2018 USD ($)	Jul. 31, 2017 USD ($)	Apr. 30, 2016 USD ($) Agreement Target AccountingUnit	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) Target	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Type of Revenue [Extensible List]												us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				277,980,000				305,263,000				$ 277,980,000	$ 305,263,000
Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid	$ 4,000,000											4,000,000
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received	21,000,000
Revenue adjustment of milestone payment recognized						$ 9,900,000
Revenue recognized from collaborative arrangement												$ 19,000,000	$ 19,400,000	$ 3,300,000
Type of Revenue [Extensible List]												us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
AbbVie Ireland Unlimited Company \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 30,000,000
Milestone payment received			14,000,000
Total transaction price			39,800,000
Estimated sublicense fees			$ 4,200,000
Number of accounting units \| AccountingUnit			2
AbbVie Ireland Unlimited Company \| Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements \| Agreement			2
Percentage of net profits or net losses related to development costs			35.00%
Amount due from agreement				0				0				$ 0	$ 0
AbbVie Ireland Unlimited Company \| CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 20,000,000
Milestone payment received	21,000,000	$ 14,000,000
Sublicense fees paid		$ 1,000,000
Accrued sublicense fees				1,100,000				500,000				1,100,000	500,000
Deferred revenue				23,200,000				11,200,000				$ 23,200,000	11,200,000
AbbVie Ireland Unlimited Company \| CD71 Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			20,000,000
Total transaction price			29,800,000
Milestone payment received			$ 14,000,000
AbbVie Ireland Unlimited Company \| CD71 Agreement \| U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales and royalties on ex U.S. sales			35.00%
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones			$ 470,000,000
AbbVie Ireland Unlimited Company \| Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid	$ 4,000,000
AbbVie Ireland Unlimited Company \| Seattle Genetics Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid			1,000,000
AbbVie Ireland Unlimited Company \| Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			10,000,000
Number of targets selected \| Target											1
Revenue recognition upon performance obligation			10,000,000
Estimated research service period												5 years
Deferred revenue				$ 4,700,000				$ 6,800,000				$ 4,700,000	$ 6,800,000
AbbVie Ireland Unlimited Company \| Discovery Agreement \| ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			$ 10,000,000
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target			2
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 275,000,000
AbbVie Ireland Unlimited Company \| Sublicense Agreement \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment percentage			5.00%

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details1)	Dec. 31, 2018
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2019-01-01
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognition upon performance obligation, service period	5 years

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)		1 Months Ended	3 Months Ended								12 Months Ended
	Sep. 29, 2017 USD ($) Target	Oct. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, value of shares issued in connection with agreement											$ 134,596,000	$ 19,957,000
Accrued liabilities			$ 26,724,000				$ 16,382,000				26,724,000	16,382,000
Revenue recognized from collaborative arrangement			11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			277,980,000				305,263,000				$ 277,980,000	$ 305,263,000
Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			40,700,000								40,700,000
Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			3,800,000								3,800,000
Collaboration and License Agreement \| Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 40,000,000
Common stock, shares issuable under agreement \| shares		1,156,069
Common stock, shares issuable under agreement, price per share \| $ / shares		$ 17.30
Common stock, value of shares issued in connection with agreement		$ 20,000,000
Period used to calculate weighted average price per share		20 days
Lock-up period for share disposal		6 months
Estimated premium on the stock sold		$ 500,000
Number of targets selected \| Target	1
Number of additional collaboration target \| Target	2
Total transaction price	$ 51,200,000
Portion of transaction price allocated to premium on sale of common stock	500,000
Estimated fair value of products	10,700,000
Revenue recognized from collaborative arrangement											$ 4,900,000	$ 1,300,000	$ 0
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			0				0				$ 0	$ 0
Collaboration and License Agreement \| Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total transaction price			46,400,000								$ 46,400,000
Estimated projected costs and research service period											7 years
Revenue recognition upon performance obligation			4,800,000								$ 4,800,000
Estimated research service period											6 years
Deferred revenue			40,700,000				45,300,000				$ 40,700,000	45,300,000
Collaboration and License Agreement \| Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			3,800,000				4,600,000				3,800,000	4,600,000
Collaboration and License Agreement \| Amgen Inc \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 950,000,000
Number of targets \| Target	3
Contingent payments payable	$ 203,000,000
Collaboration and License Agreement \| Amgen Inc \| Maximum \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 455,000,000
Percentage share of profit and losses	50.00%
Sublicense Agreement \| Amgen Inc \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment percentage	15.00%
Accrued liabilities			$ 2,100,000				$ 2,100,000				$ 2,100,000	$ 2,100,000

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)				3 Months Ended								12 Months Ended			55 Months Ended
	Apr. 25, 2017 USD ($) Target SaleIndicator	Jul. 07, 2014 USD ($) Target Term ResearchTarget SaleIndicator	May 23, 2014 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2017 USD ($)	Mar. 17, 2017 Target	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Type of Revenue [Extensible List]												us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				277,980,000				305,263,000				$ 277,980,000	$ 305,263,000		$ 277,980,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				205,600,000								$ 205,600,000			205,600,000
Collaborative Arrangement \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration target research term		2 years
Number of additional collaboration target \| Target		2
Research terms												Each collaboration target has a two-year research term and the two additional targets must be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target can be extended in one year increments up to three times.
Extension of research term for each collaboration target		1 year
Upfront payment received	$ 200,000,000	$ 50,000,000	$ 250,000,000
Contingent milestone payments receivable														$ 15,000,000				$ 10,000,000
Revenue recognition upon performance obligation			272,800,000
Upfront payment received			$ 10,800,000
Milestone payment received															12,000,000
Estimated research service termination date												Apr. 25, 2023
Revenue recognized from collaborative arrangement												$ 32,800,000	$ 36,500,000	$ 9,600,000
Type of Revenue [Extensible List]												us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				205,600,000				235,000,000				$ 205,600,000	$ 235,000,000		205,600,000
Amount due from agreement				$ 100,000				$ 10,100,000				$ 100,000	$ 10,100,000		$ 100,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Clinical Candidate
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable														$ 2,000,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| New Drug Application for CTLA-4
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable																$ 10,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target \| Target		4
Number of collaboration target \| Target		2
Period of nomination of additional target from effective date		5 years
Times of increments for extended collaboration target research time \| Term		3
Aggregate future contingent milestone payment		$ 1,217,000,000												$ 3,586,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		25,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achievement Of Development Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	116,000,000	114,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achievement Of Development Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 456,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 124,000,000	$ 124,000,000
Number of sales indicators \| SaleIndicator	3	3
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving First Commercial Sale If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 496,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 60,000,000	$ 60,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 240,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achievement of Development Milestones if Eight Research Targets Selected by Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 928,000,000
Number of research targets selected \| Target	8
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Eight Targets Selected for the First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 992,000,000
Number of research targets selected \| Target	8
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestones if Eight Research Targets Selected for First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 480,000,000
Number of research targets selected \| Target	8
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Development Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 56,300,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 450,000,000
Number of research targets selected \| Target	8
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Milestone Payments for First Commercial Sale in Various Territories for Up to Three Indication
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 62,000,000
Number of sales indicators \| SaleIndicator	3
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 496,000,000
Number of research targets selected \| Target	8
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 30,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Sales Milestone Payment if Eight Research Target are selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 240,000,000
Number of research targets selected \| Target	8
Collaboration and License Agreement \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of non-oncology target \| Target																	2
Collaboration and License Agreement \| Maximum \| Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target \| Target																	6

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - USD ($)	1 Months Ended		3 Months Ended								12 Months Ended
	Aug. 31, 2017	Jan. 31, 2014	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			$ 277,980,000				$ 305,263,000				$ 277,980,000	$ 305,263,000
Revenue recognized from collaborative arrangement			11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Collaborative Arrangement \| Immuno Gen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		$ 0
Milestones payments made during period	$ 1,000,000
Milestone payments accrued			0								$ 0
Estimated total fair value consideration recorded as deferred revenue		13,200,000	0				700,000				0	$ 700,000
Estimated fair value of assets and services		13,200,000
Revenue recognized from collaborative arrangement											$ 1,500,000	$ 12,500,000	$ 0
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Amount due from agreement			$ 0				$ 0				$ 0	$ 0
Collaborative Arrangement \| Immuno Gen Inc \| Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services		12,700,000
Collaborative Arrangement \| Immuno Gen Inc \| Research Services, Joint Research Committee Participation and Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services		500,000
Collaborative Arrangement \| Immuno Gen Inc \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development and regulatory milestones		30,000,000
Contingent payments receivable upon achieving sales milestones		50,000,000
Contingent payments payable upon achieving development and regulatory milestones		60,000,000
Contingent payments payable upon achieving sales milestones		$ 100,000,000

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details)	1 Months Ended		3 Months Ended								12 Months Ended			68 Months Ended
	Dec. 31, 2014 USD ($)	May 31, 2013 USD ($) Target	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional target as deferred revenue											$ 31,900,000
Remaining deferred revenue recognized											47,477,000
Revenue recognized from collaborative arrangement			$ 11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			277,980,000				305,263,000				$ 277,980,000	$ 305,263,000		$ 277,980,000
Collaborative Arrangement \| Pfizer Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target		2
Nominated number of additional research targets \| Target		2
Additional target as deferred revenue	$ 1,500,000
Remaining deferred revenue recognized						$ 1,100,000
Upfront payment received		$ 6,000,000
Adjusted amortization period of deferred revenue														5 years 6 months
Revenue recognized from collaborative arrangement											$ 1,400,000	$ 1,900,000	$ 2,200,000
Type of Revenue [Extensible List]											us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			0				1,600,000				$ 0	$ 1,600,000		$ 0
Amount due from agreement			$ 0				$ 13,000				$ 0	$ 13,000		$ 0

Research and Collaboration Agreements - Summary of Contract Liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Revenue From Contract With Customer [Abstract]
Balance as of December 31, 2017	$ 305,263
ASC 606 Adoption Adjustment	10,912
Additions, Adjustment to Transaction Price from Performance Obligation Satisfied	21,000
Deductions to Revenue Recognized from an Adjustment to Transaction Price During the Period	(11,718)
Deductions to Revenue Recognized from Amounts Included in Contract Liability at the Beginning of the Period	(47,477)
Balance as of December 31, 2018	$ 277,980

Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details) - USD ($) $ in Thousands		1 Months Ended		12 Months Ended
	May 31, 2018	May 31, 2018	Jul. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Additional target as deferred revenue				$ 31,900
ASC 606 Adoption Adjustment				10,912
Revenue recognized based on estimated percentage completed to-date				11,718
Deductions to deferred revenue recognized during the period				47,477
Deferred revenue				277,980	$ 305,263
Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 40,700
Deferred revenue recognition maturity date				Sep. 30, 2024
Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 3,800
Deferred revenue recognition maturity date				Sep. 30, 2023
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 205,600
Deferred revenue recognition maturity date				Apr. 30, 2025
CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
ASC 606 Adoption Adjustment				$ 10,900
Adjustment to transaction price from performance obligation satisfied	$ 21,000			21,000
Revenue recognized based on estimated percentage completed to-date				11,700
CD71 Agreement \| AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid			$ 1,000
Deferred revenue				$ 23,200	11,200
Deferred revenue recognition maturity date				Apr. 30, 2021
Seattle Genetics Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid		$ 4,000		$ 4,000
Seattle Genetics Agreement \| AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sublicense fees paid		$ 4,000
Discovery Agreement \| AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				$ 4,700	$ 6,800
Deferred revenue recognition maturity date				Apr. 30, 2021

License Agreement - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	45,083,209	38,478,560
Accrued amounts included in current liabilities	$ 26,724,000	$ 16,382,000
Annual minimum royalty obligations	$ 150,000
License termination period	60 days
UCSB
License Agreement [Line Items]
Common stock, shares issued				157,332
Milestone and minimum annual royalty provision paid	$ 600,000	13,500,000	$ 2,100,000
Accrued amounts included in current liabilities	$ 3,200,000	$ 2,600,000

Commitments and Contingencies - Additional Information (Details)		12 Months Ended
	Dec. 10, 2015 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Operating Leased Assets [Line Items]
Rent expense		$ 4,200,000	$ 4,200,000	$ 1,800,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount		900,000
Restricted cash		$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space \| ft²	76,000
Lease term description		The term of the Lease commenced on October 1, 2016. The 2016 Lease has an initial term of ten years from the commencement date, and the Company has an option to extend the initial term for an additional five years at the then fair rental value as determined pursuant to the 2016 Lease.
Initial lease term		10 years
Extended lease term		5 years
Maximum one-time improvement allowance	$ 12,600,000
Improvement allowance from recoverable rent	2,300,000
New Lease Agreement \| First year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	3,100,000
New Lease Agreement \| Second twelve months of lease term
Operating Leased Assets [Line Items]
Annual lease rent	4,300,000
New Lease Agreement \| Tenth year of lease term
Operating Leased Assets [Line Items]
Annual lease rent	$ 5,500,000

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2019	$ 4,854
2020	4,990
2021	5,129
2022	5,273
2023 and beyond	21,109
Total	$ 41,355

Common Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Oct. 14, 2015
Equity [Abstract]
Common stock, shares authorized	75,000,000	75,000,000	75,000,000
Common stock, par value	$ 0.00001	$ 0.00001	$ 0.00001
Preferred stock, shares authorized			10,000,000
Preferred stock, par value			$ 0.00001
Dividends on common stock	$ 0	$ 0

Common Stock - Reserved Shares of Common Stock Issuance on Converted Basis (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Options issued and outstanding	7,803,773	6,503,458
Shares available for future stock option grants	1,884,494	2,324,793
Shares available for future employee stock purchase plan	1,301,254	980,389
Total	10,989,521	9,808,640

Stock-based Compensation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Jan. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		10,989,521	9,808,640
Stock option expiration period		10 years
Aggregate intrinsic value of stock options exercised		$ 14,600	$ 10,500	$ 4,600
Weighted average grant date fair value, options granted		$ 14.205	$ 8.207	$ 8.936
Unrecognized compensation expense, options granted to employees		$ 34,600
Number of shares issued during period		63,920	67,746
Stock-based compensation expense		$ 16,878	$ 11,287	$ 10,095
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		1,301,254
Estimated fair value of the shares on the date of grant		85.00%
Unrecognized stock-based compensation cost, period for recognition		5 months
Plan maximum percentage of eligible compensation to purchase shares		15.00%
Stock-based compensation expense		$ 500	200	100
Unrecognized stock-based compensation cost		$ 300	100
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized stock-based compensation cost, period for recognition		2 years 4 months 24 days
Stock-based compensation expense		$ 16,700	11,000	9,400
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 200	$ 300	$ 900
Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		2.08%
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance		1,884,494			2,444,735
Increase in number of shares available for issuance, as proportion of issued and outstanding shares of common stock, percentage	4.00%
Stock option expiration period		10 years
Employees holding voting rights of all classes of stock, percentage		10.00%
2015 Plan \| Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		4 years
Options granted generally vest percent		25.00%
Early Exercise of Employee Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest period		36 months
Early Exercise of Employee Options \| Share Based Compensation Award Tranche One
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent		25.00%
Minimum \| 2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Estimated fair value of the shares on the date of grant		110.00%

Stock-based Compensation - Schedule of Activity Under Company's Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Options Available for Grant
Balances, beginning of the period	2,324,793
Balances, end of the period	1,884,494	2,324,793
Number of Options
Balances, beginning of the period	6,503,458	6,158,746	5,270,751
Options granted	2,127,400	2,138,620	1,356,546
Options exercised	(673,382)	(764,576)	(414,396)
Options forfeited	(153,703)	(1,029,332)	(54,155)
Balances, end of the period	7,803,773	6,503,458	6,158,746
Options Exercisable, end of the period	4,520,806
Options vested and expected to vest, end of the period	7,803,773
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 8.157	$ 3.694	$ 1.197
Options granted	24.654	13.566	13.234
Options exercised	6.172	4.140	1.549
Options forfeited	14.293	9.118	4.578
Balances, end of the period	12.622	$ 8.157	$ 3.694
Options Exercisable, end of the period	7.853
Options vested and expected to vest, end of the period	$ 12.622
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	7 years 3 months 18 days
Options Exercisable, end of the period	6 years 3 months 18 days
Options vested and expected to vest, end of the period	7 years 3 months 18 days
Aggregate Intrinsic Value
Balances, end of the period	$ 40,901
Options Exercisable—December 31, 2018	36,017
Options vested and expected to vest—December 31, 2018	$ 40,901
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	2,324,793	2,493,188	2,401,406
Options authorized	1,539,142	1,459,606	1,441,328
Options granted	(2,127,400)	(2,138,620)	(1,367,546)
Options forfeited	147,959	510,619	18,000
Balances, end of the period	1,884,494	2,324,793	2,493,188

Stock-based Compensation - Schedule of Stock Option Outstanding and Exercisable (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options Outstanding, Number \| shares	7,803,773
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	7 years 3 months 18 days
Options Exercisable, Number \| shares	4,520,806
Options Exercisable, Weighted- Average Exercise Price	$ 7.853
$0.945 - $1.386
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	0.945
Range of Exercise Price, upper limit	$ 1.386
Options Outstanding, Number \| shares	820,235
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	3 years 4 months 24 days
Options Exercisable, Number \| shares	820,235
Options Exercisable, Weighted- Average Exercise Price	$ 1.174
$1.449 - $1.575
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	1.449
Range of Exercise Price, upper limit	$ 1.575
Options Outstanding, Number \| shares	920,347
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	5 years 10 months 24 days
Options Exercisable, Number \| shares	915,120
Options Exercisable, Weighted- Average Exercise Price	$ 1.545
$4.473 - $4.473
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	4.473
Range of Exercise Price, upper limit	$ 4.473
Options Outstanding, Number \| shares	119,531
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	6 years 4 months 24 days
Options Exercisable, Number \| shares	119,531
Options Exercisable, Weighted- Average Exercise Price	$ 4.473
$6.615 - $6.615
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	6.615
Range of Exercise Price, upper limit	$ 6.615
Options Outstanding, Number \| shares	1,171,404
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	6 years 7 months 6 days
Options Exercisable, Number \| shares	910,002
Options Exercisable, Weighted- Average Exercise Price	$ 6.615
$10.170 - $11.640
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	10.170
Range of Exercise Price, upper limit	$ 11.640
Options Outstanding, Number \| shares	497,541
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	7 years 8 months 12 days
Options Exercisable, Number \| shares	298,494
Options Exercisable, Weighted- Average Exercise Price	$ 10.715
$11.940 - $11.940
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	11.940
Range of Exercise Price, upper limit	$ 11.940
Options Outstanding, Number \| shares	842,500
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	8 years 1 month 6 days
Options Exercisable, Number \| shares	403,695
Options Exercisable, Weighted- Average Exercise Price	$ 11.940
$12.000 - $14.460
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	12.000
Range of Exercise Price, upper limit	$ 14.460
Options Outstanding, Number \| shares	1,073,581
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	7 years 6 months
Options Exercisable, Number \| shares	648,933
Options Exercisable, Weighted- Average Exercise Price	$ 14.172
$14.620 - $25.670
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	14.620
Range of Exercise Price, upper limit	$ 25.670
Options Outstanding, Number \| shares	846,434
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	9 years 1 month 6 days
Options Exercisable, Number \| shares	157,099
Options Exercisable, Weighted- Average Exercise Price	$ 18.574
$25.820 - $25.820
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	25.820
Range of Exercise Price, upper limit	$ 25.820
Options Outstanding, Number \| shares	885,000
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	9 years 1 month 6 days
Options Exercisable, Number \| shares	202,809
Options Exercisable, Weighted- Average Exercise Price	$ 25.820
$26.300 - $29.710
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Range of Exercise Price, lower limit	26.300
Range of Exercise Price, upper limit	$ 29.710
Options Outstanding, Number \| shares	627,200
Options Outstanding, Weighted-Average Remaining Contractual Life (Years)	9 years
Options Exercisable, Number \| shares	44,888
Options Exercisable, Weighted- Average Exercise Price	$ 26.740

Stock-based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation expense:
Stock-based compensation expense	$ 16,878	$ 11,287	$ 10,095
Research and development
Stock-based compensation expense:
Stock-based compensation expense	8,313	5,161	4,925
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 8,565	$ 6,126	$ 5,170

Stock-based Compensation - Schedule of Estimated Fair Value of Employee Stock Options and ESPP Using Black-Scholes Valuation Model (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	65.60%	69.10%	76.40%
Expected volatility, maximum	69.30%	72.40%	83.50%
Risk-free interest rate, minimum	2.50%	1.70%	1.20%
Risk-free interest rate, maximum	3.00%	2.20%	2.10%
Employee Stock Option \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	4 years 8 months 12 days	4 years 10 months 24 days	5 years 3 months 18 days
Employee Stock Option \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	4 years 10 months 24 days	5 years 3 months 18 days	5 years 10 months 24 days
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	46.40%	52.30%	50.40%
Expected volatility, maximum	70.50%	63.80%	75.60%
Risk-free interest rate, minimum	2.10%	1.10%	0.50%
Risk-free interest rate, maximum	2.60%	1.50%	0.60%
Expected term (in years)	6 months	6 months	6 months

Income Taxes - Schedule of Components of Provision for (Benefit from) Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal		$ 14,302
State		1	$ 1	$ 1
Total current		14,303	1	1
Deferred:
Federal			(514)	(20)
Total deferred			(514)	(20)
Provision for (benefit from) income taxes	$ 8,912	$ 14,303	$ (513)	$ (19)

Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
U.S. federal taxes at statutory rate	21.00%	34.00%	34.00%
State tax, net of federal benefit	0.70%	7.60%	0.80%
Stock compensation	1.70%	2.30%	(0.60%)
Tax attributes subject to 382 limitation	0.00%	27.50%	0.00%
Tax credits	6.20%	2.70%	2.20%
Change in valuation allowance	(49.50%)	(9.30%)	(35.60%)
Change in deferreds due to rate change	0.00%	(58.60%)	0.00%
Other	(0.50%)	(5.00%)	(0.80%)
Total	(20.40%)	1.20%	0.00%

Income Taxes - Schedule of Deferred Income Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 15,468	$ 24,682	$ 24,528
Research and development credits	7,514	5,757	2,683
Intangible—in-process R&D			81
Deferred revenue	56,881	15,631	13,857
Accruals and deferred rent	1,955	1,256	1,335
Stock-based compensation	5,746	3,831	3,963
Other	39	32	1
Total gross deferred income tax assets	87,603	51,189	46,448
Less: valuation allowance	(86,466)	(50,791)	(46,137)
Deferred tax assets, net of valuation allowance	1,137	398	311
Fixed assets	(854)	(282)	(229)
In-process R&D			(595)
Intangible assets	(108)	(116)
Prepaid Expenses	(175)
Deferred tax liabilities	$ (1,137)	$ (398)	(824)
Net deferred income tax liabilities			$ (513)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Deferred tax asset, net change in total valuation allowance increase during period	$ 35,700,000	$ 4,700,000	$ 21,100,000
Federal statutory income tax rate	21.00%	34.00%	34.00%
Change in deferred tax asset due to tax cuts and jobs act of 2017	$ 25,700,000
Impact of adoption of new accounting pronouncements	(10,912,000)
Unrecognized tax benefits	3,756,000	$ 4,320,000	$ 1,182,000	$ 666,000
Unrecognized tax benefits would affect company's effective tax rate	1,000,000	0
Interest and penalties accrued	0	$ 0	$ 0
Accumulated Deficit
Income Tax Contingency [Line Items]
Impact of adoption of new accounting pronouncements	(10,912,000)
Accumulated Deficit \| ASC 606
Income Tax Contingency [Line Items]
Impact of adoption of new accounting pronouncements	10,900,000
Maximum
Income Tax Contingency [Line Items]
Federal statutory income tax rate		35.00%
Minimum
Income Tax Contingency [Line Items]
Percentage of change in ownership		50.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 65,600,000
Operating loss carryforwards expiration year	2034
Open tax year	2010 2011 2012 2013 2014 2015 2016 2017 2018
Federal \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 5,700,000
Tax credit carryforward, expiration	2031
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 24,200,000
Operating loss carryforwards expiration year	2033
Open tax year	2010 2011 2012 2013 2014 2015 2016 2017 2018
State \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 5,500,000

Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at the beginning of the year	$ 4,320	$ 1,182	$ 666
Additions based on tax positions related to current year	1,166	521	23
Adjustment based on tax positions related to prior years	(1,730)	2,617	493
Balance at end of the year	$ 3,756	$ 4,320	$ 1,182

Related Party Transactions - Additional Information (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) BoardMember	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) BoardMember	Dec. 31, 2016 USD ($)
Related Party Transaction [Line Items]
Revenue recognized	$ 11,471,000	$ 12,509,000	$ 21,338,000	$ 14,184,000	$ 27,074,000	$ 24,144,000	$ 8,752,000	$ 11,653,000	$ 59,502,000	$ 71,623,000	$ 12,845,000
Collaborative Arrangement \| Pfizer Inc
Related Party Transaction [Line Items]
Threshold limit to be qualified as related party										10.00%
Revenue recognized									$ 1,400,000	$ 1,900,000	2,200,000
Deferred revenue											3,400,000
Amount due from agreement											100,000
Third Rock Ventures
Related Party Transaction [Line Items]
Threshold limit to be qualified as related party										10.00%
Number of board members affiliated with related party \| BoardMember					1					1
General and administrative expense from transactions with related party										$ 35,000	48,000
Accounts payable, related parties					$ 0					$ 0	$ 12,000

Supplementary Data - Quarterly Financial Data (Unaudited) - Unaudited Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 11,471	$ 12,509	$ 21,338	$ 14,184	$ 27,074	$ 24,144	$ 8,752	$ 11,653	$ 59,502	$ 71,623	$ 12,845
Net income (loss)	$ (32,233)	$ (23,431)	$ (13,447)	$ (15,493)	$ 621	$ (10,247)	$ (25,216)	$ (8,257)	$ (84,604)	$ (43,099)	$ (58,900)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.72)	$ (0.53)	$ (0.35)	$ (0.40)	$ 0.02	$ (0.28)	$ (0.69)	$ (0.23)	$ (2.03)	$ (1.16)	$ (1.63)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Research and Collaboration Agreements - MD Anderson - Additional Information (Details)	Nov. 02, 2015
MD Anderson
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option expiration date	Nov. 01, 2018

Defined Contribution Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Company contributions to defined contribution plan	$ 590,000	$ 255,000	$ 201,000

Fair Value Measurements and Short-Term Investments - Additional Information (Details)	Dec. 31, 2018 USD ($)
Fair Value Disclosures [Abstract]
Contractual maturities of longer than one year	$ 0

Subsequent Event - Additional Information (Details)	12 Months Ended
Subsequent Event - Additional Information (Details)	Dec. 31, 2018
Subsequent Events [Abstract]
License termination period	60 days