Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2017	Mar. 05, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
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	Accelerated Filer
Entity Public Float			$ 360.5
Entity Common Stock, Shares Outstanding		38,611,158

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 177,548	$ 104,645
Short-term investments	196,562	77,293
Accounts receivable	10,139	2,159
Related party accounts receivable		154
Prepaid expenses and other current assets	4,352	3,896
Total current assets	388,601	188,147
Property and equipment, net	4,218	4,392
Intangible assets, net	1,604	1,750
Goodwill	949	949
Restricted cash	917	917
Other assets	1,355	2,973
Total assets	397,644	199,128
Current liabilities:
Accounts payable	4,205	6,596
Accrued liabilities	16,383	8,824
Deferred revenues, current portion	40,559	20,347
Total current liabilities	61,147	35,767
Deferred revenue, net of current portion	264,704	83,803
Deferred tax liability		513
Other long-term liabilities	1,897	566
Total liabilities	327,748	120,649
Commitments and contingencies (Note 10)
Stockholders' equity
Common stock, $0.00001 par value; 75,000,000 shares authorized at December 31, 2017 and 2016; 38,478,560 and 36,490,169 shares issued and outstanding at December 31, 2017 and 2016, respectively	1	1
Additional paid-in capital	289,454	254,871
Accumulated other comprehensive loss	(94)	(27)
Accumulated deficit	(219,465)	(176,366)
Total stockholders' equity	69,896	78,479
Total liabilities, convertible preferred stock and stockholders' equity	397,644	199,128
Convertible Preferred Stock
Stockholders' equity
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized at December 31, 2017 and 2016; no shares issued and outstanding at December 31, 2017 and 2016, respectively

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	38,478,560	36,490,169
Common stock, shares outstanding	38,478,560	36,490,169
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 71,623	$ 12,845	$ 5,941
Revenues from related parties		2,198	1,771
Total revenues	71,623	15,043	7,712
Operating expenses:
Research and development	92,277	54,755	28,357
General and administrative	25,605	19,874	12,558
Total operating expenses	117,882	74,629	40,915
Loss from operations	(46,259)	(59,586)	(33,203)
Interest income	2,674	736	1,315
Interest expense			(1,732)
Other income (expense), net	(27)	(69)	(1,744)
Loss before provision for (benefit from) income taxes	(43,612)	(58,919)	(35,364)
Provision for (benefit from) income taxes	(513)	(19)	10
Net loss	(43,099)	(58,900)	(35,374)
Accretion to redemption value and cumulative dividends on preferred stock			(6,705)
Net loss attributable to common stockholders	$ (43,099)	$ (58,900)	$ (42,079)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.16)	$ (1.63)	$ (4.90)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	37,166,830	36,234,732	8,595,247
Other comprehensive loss:
Changes in unrealized gain (losses) on investments	$ (67)	$ 49	$ (76)
Comprehensive loss	$ (43,166)	$ (58,851)	$ (35,450)

STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Stockholder Notes	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock Series C	Redeemable Convertible Preferred Stock Series B-1	Redeemable Convertible Preferred Stock Series D	Convertible Preferred Stock
Temporary equity, beginning balance at Dec. 31, 2014							$ 76,236				$ 474
Temporary equity, beginning balance, shares at Dec. 31, 2014							18,458,289				244,782
Beginning balance, shares at Dec. 31, 2014		996,520
Beginning balance at Dec. 31, 2014	$ (78,541)	$ 1	$ (404)			$ (78,138)
Issuance of preferred stock, net of issuance costs								$ 4,969		$ 69,744
Issuance of preferred stock, net of issuance costs, shares								941,842		7,490,540
Issuance of preferred stock upon net exercise of warrants, shares									60,640
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering	159,163			$ 159,163			$ (159,163)
Conversion of redeemable convertible preferred stock to common stock in connection with initial public offering, shares		26,951,311					(26,951,311)
Conversion of convertible preferred stock to common stock in connection with initial public offering	474			474							$ (474)
Conversion of convertible preferred stock to common stock in connection with initial public offering, shares		244,782									(244,782)
Issuance of common stock	81,764			81,764
Issuance of common stock, shares		7,666,667
Extinguishment of preferred stock liability							$ 1,509
Extinguishment of preferred stock warrant liability	788			788
Exercise of stock options	$ 263			263
Exercise of stock options, shares	173,929	173,929
Interest on stockholder notes	$ (4)		(4)
Repayment on stockholders note	330		330
Stock-based compensation	3,986			3,986
Accretion to redemption value and cumulative dividends on preferred stock							$ 6,705
Accretion to redemption value and cumulative dividends on preferred stock	(6,705)			(2,751)		(3,954)
Other comprehensive income (loss)	(76)				$ (76)
Net loss	(35,374)					(35,374)
Ending balance, shares at Dec. 31, 2015		36,033,209
Ending balance at Dec. 31, 2015	126,068	$ 1	(78)	243,687	(76)	(117,466)
Issuance of common stock	159			159
Issuance of common stock, shares		11,000
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	31,564
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)
Temporary equity, ending balance, shares at Dec. 31, 2016	0
Ending balance, shares at Dec. 31, 2016		36,490,169
Ending balance at Dec. 31, 2016	$ 78,479	$ 1		254,871	(27)	(176,366)
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)
Temporary equity, ending balance, shares at Dec. 31, 2017	0
Ending balance, shares at Dec. 31, 2017		38,478,560
Ending balance at Dec. 31, 2017	$ 69,896	$ 1		$ 289,454	$ (94)	$ (219,465)

STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Offering cost for common stock issued in public offering	$ 3,796
Underwriting discount for common stock issued in public offering	6,440
Redeemable Convertible Preferred Stock \| Series C
Temporary equity stock issued issuance costs	30
Redeemable Convertible Preferred Stock \| Series D
Temporary equity stock issued issuance costs	$ 255

STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (43,099)	$ (58,900)	$ (35,374)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Loss/(gain) on disposal of property and equipment	17	(47)	25
Depreciation and amortization	1,645	1,733	1,206
Amortization of debt discount			80
Accretion of discount on short-term investments	371	1,662	1,186
Stock-based compensation expense	11,287	10,095	3,986
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	8
Changes in operating assets and liabilities
Accounts receivable	(7,980)	(1,787)	1,131
Related party accounts receivable	154	218
Prepaid expenses and other current assets	(456)	(1,597)	(1,491)
Other assets	1,618	(2,609)	128
Accounts payable	(2,441)	1,765	2,944
Deferred revenue	189,913	43,317	(6,130)
Accrued liabilities and other liabilities	9,157	3,953	3,170
Net cash provided by/(used in) operating activities	170,373	(2,032)	(27,415)
Cash flows from investing activities:
Purchases of property and equipment	(1,559)	(2,176)	(1,594)
Proceeds from sales of assets		52
Purchases of investments	(218,707)	(121,517)	(250,901)
Maturities of investments	99,000	169,500	122,750
Increase in restricted cash			(817)
Net cash (used in)/provided by investing activities	(121,266)	45,859	(130,562)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			74,430
Proceeds from issuance of common stock, net of issuance costs	19,957
Proceeds from initial public offering, net of issuance costs			81,777
Repayment of notes payable			(3,067)
Payment of deferred offering costs		(12)
Net cash provided by financing activities	23,796	996	153,403
Proceeds from employee stock purchases and exercise of stock options	3,839	930	263
Proceeds from stockholder notes		78
Net increase/(decrease) in cash and cash equivalents	72,903	44,823	(4,574)
Cash and cash equivalents, beginning of year	104,645	59,822	64,396
Cash and cash equivalents, end of year	177,548	104,645	59,822
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 361	$ 473	100
Accretion to redemption value and cumulative dividends on preferred stock			6,705
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock			1,509
Stock issuance costs in accounts payable and accrued liabilities			13
Convertible Preferred Stock Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability			1,114
Convertible Preferred Stock Warrant Liability
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
Change in fair value of liability			$ 602

Description of the Business	12 Months Ended
Description of the Business	Dec. 31, 2017
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 pioneering a novel class of investigational antibody therapeutics based on its Probody therapeutic technology platform. The Company is located in South San Francisco, California and was incorporated in the state of Delaware in September 2010. Initial Public Offering On October 7, 2015, the Company’s registration statement on Form S-1 relating to its initial public offering (“IPO”) of its common stock was declared effective by the Securities and Exchange Commission (“SEC”) and the shares of its common stock began trading on The NASDAQ Global Select Market on October 8, 2015. The public offering price of the shares sold in the IPO was $12.00 per share. The IPO closed on October 14, 2015, pursuant to which the Company sold 7,666,667 shares of common stock, including the sale of 1,000,000 shares of common stock to the underwriters upon their exercise of their option to purchase additional shares. The Company received net proceeds of approximately $81.8 million, after underwriting discounts, commissions and estimated offering expenses. Immediately prior to the consummation of the IPO, all outstanding shares of convertible preferred stock and redeemable convertible preferred stock converted into common stock. 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, par value $0.00001 (“Common Stock”), for an aggregate cash purchase price of $20 million. The Shares are to be 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.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

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:

	Revenue						Accounts Receivable, net
	For the Year Ended December 31,						December 31,
	2017		2016		2015		2017		2016
AbbVie Ireland Unlimited Company	$	19,434	$	3,268	$	—	**		$	—
Bristol-Myers Squibb Company		36,492		9,577		5,941		10,126		2,159
ImmunoGen, Inc.		12,503		—		—		—		—
Pfizer Inc.	*			2,198		1,771	**		**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	68,429	$	15,043	$	7,712	$	10,126	$	2,159

* 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.

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, 2017, 2016 and 2015.

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, 2017, 2016 and 2015.

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.

Convertible Preferred Stock Warrant Liability

Freestanding warrants for shares that are contingently redeemable are classified as liabilities on the balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are subject to re-measurement at each balance sheet date and the change in fair value, if any, is included in other income (expense), net. The Company adjusted the liability for changes in fair value until the consummation of its IPO in October 2015, at which time all convertible preferred stock warrants were net exercised into shares of common stock and the related convertible preferred stock warrant liability was reclassified to additional paid-in capital.

Convertible Preferred Stock Liability

The obligation to issue additional shares of Series B-1 and Series C redeemable convertible preferred stock at a future date pursuant to certain preferred stock purchase agreements entered into prior to the date of the IPO, was determined to be a freestanding instrument that should be accounted for as a liability. At initial recognition, the Company recorded the convertible preferred stock liability on the balance sheets at its estimated fair value. The liability was subject to remeasurement at each balance sheet date, with changes in fair value recognized as a component of other income (expense), net. At the time of each funding, the Company remeasured the liability, with the change in fair value recognized as a component of other income (expense), net and then reclassified the fair value associated with the convertible preferred stock liability to the applicable series of redeemable convertible preferred stock. Immediately prior to the consummation of the Company’s IPO in October 2015, the convertible preferred stock converted to 27,135,453 shares of common stock.

Comprehensive Income (Loss)

Comprehensive income (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.

Revenue Recognition

The Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; the price to the customer is fixed or determinable; and collectability is reasonably assured.

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 arrangements involving the delivery of more than one element, each required deliverable is evaluated to determine whether it qualifies as a separate unit of accounting. The determination is based on whether the deliverable has “standalone value” to the customer. If a deliverable does not qualify as a separate unit of accounting, it is combined with the other applicable undelivered item(s) within the arrangement and these combined deliverables are treated as a single unit of accounting.

The arrangement’s consideration that is fixed or determinable is allocated to each separate unit of accounting based on the relative selling price methodology in accordance with the selling price hierarchy, which includes vendor-specific objective evidence (“VSOE”) of selling price, if available, or third-party evidence of selling price if VSOE is not available, or the best estimate of selling price, if neither VSOE nor third-party evidence is available.

Payments or reimbursements for the Company’s research and development efforts for the arrangements where such efforts are considered as deliverables are recognized as the services are performed and are presented on a gross basis. When upfront payments are received and if there is no discernible pattern of performance and/or objectively measurable performance measures do not exist, the Company recognizes revenue ratably over the associated period of performance.

The Company’s collaboration and license agreements may include contingent payments related to specified research, development and regulatory milestones and sales-based 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, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. Each contingent and milestone payment is evaluated to determine whether it is substantive and at risk to both parties. The Company recognizes any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. Any payments that are contingent upon achievement of a non-substantive milestone are recognized as revenue prospectively, when such payments become due and collectible, over the remaining expected performance period under the arrangement, which is generally the remaining period over which the research and development services are expected to be provided.

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, the Company records forfeitures 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 under the asset and liability method which requires, among other things, that deferred income taxes be provided for temporary differences between the tax basis of the Company’s assets and liabilities and their financial statement reported amounts. Management makes estimates, assumptions and judgments to determine the Company’s provision for income taxes and also for deferred tax assets and liabilities, and any valuation allowances recorded against the Company’s deferred tax assets. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and to the extent the Company believes that recovery is not more likely than not, the Company must establish a valuation allowance

The Company has adopted ASC 740-10, Accounting for Uncertainty in Income Taxes, that prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of uncertain tax positions taken or expected to be taken in the Company’s income tax return, and also provides guidance on derecognition, classification, interest and penalties, accounting in interim periods, disclosure and transition. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

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, without consideration of potentially dilutive securities. 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 potentially dilutive securities is anti-dilutive.

Reclassifications

Certain amounts in the prior year’s Statement of Operations and Comprehensive Loss have been reclassified to conform to the current presentation. These reclassifications had no effect on previously reported net income.

Recent Accounting Pronouncements

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers, 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. The ASU will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard will be effective for the Company on January 1, 2018. The standard permits the use of either the full retrospective method, in which case the standard would be applied to each prior reporting period presented, or the modified retrospective method, in which case the cumulative effect of applying the standard would be recognized at the date of initial application. 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 have the same effective date and transition date of January 1, 2018. The Company adopted this standard on January 1, 2018 using the modified retrospective method. This approach allows the Company to apply the new standard to (1) all new contracts entered into after January 1, 2018 and (2) all existing contracts for which all (or substantially all) of the revenue has not been recognized under legacy revenue guidance as of January 1, 2018 through a cumulative adjustment to equity. Revenue presented in the Company’s comparative financial statements for periods prior to January 1, 2018 would not be revised.

The new revenue recognition standard referred to as ASC 606 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 and replaces most of the existing revenue recognition standards in U.S. GAAP. A five-step model will be utilized to achieve the core principle; (1) identify the customer contract, (2) identify the contract’s performance obligations, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations and (5) recognize revenue when or as a performance obligation is satisfied. Under ASC 606, the timing of recognizing royalties, sales-based milestones and other forms of contingent consideration is not expected to change. However, transaction prices are no longer required to be fixed or determinable and certain variable consideration might be recognized prior to the occurrence or resolution of the contingent event to the extent it is probable that a significant reversal in the amount of estimated cumulative revenue will not occur.

The Company evaluated its contracts with customers under the above ASUs (collectively, “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. For some contracts, there is no change to timing or method of recognizing revenue while for others, there are changes to timing and/or method of recognizing revenue. The Company will record adjustments upon the adoption of ASC 606 as a result of different accounting treatment of our 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 old standard, milestone payments are 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 the new standard, 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 our collaboration agreements will change under the new revenue standard.

The Company has substantially completed its analysis of the impact of adopting ASC 606 and expects the impact of adopting ASC 606 to result in an increase in accumulated deficit between $6.0 million to $10.0 million with a corresponding increase to deferred revenues.

In addition to the above impact on the financial statements, the Company will include expanded disclosures, including the disaggregation of revenue, significant judgments made with regard to revenue recognition and reconciliation of contract balances, among other disclosures.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). Under ASU 2016-2, an entity will be required to recognize right-of-use assets and lease liabilities on its balance sheet and disclose key information about leasing arrangements. ASU 2016-02 offers specific accounting guidance for a lessee, a lessor and sale and leaseback transactions. Lessees and lessors are required to disclose qualitative and quantitative information about leasing arrangements to enable a user of the financial statements to assess the amount, timing and uncertainty of cash flows arising from leases. For public companies, ASU 2016-02 is effective for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period, and requires a modified retrospective adoption, with early adoption permitted. The Company plans to adopt this guidance beginning with its first quarter ending March 31, 2019. The Company is in the process of evaluating the future impact of ASU 2016-02 on its financial statements.

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.

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 new standard is effective for fiscal years beginning after December 15, 2017 and interim periods within those fiscal years, with early adoption permitted. An entity that elects early adoption must adopt all of the amendments in the same period. The Company plans to adopt this standard in its first quarter ended March 31, 2018.

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. ASU 2016-18 is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted. The amendments in this ASU should be applied using a retrospective transition method to each period presented. The Company plans to adopt this standard in its first quarter ended March 31, 2018.

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 assessing the impact of this new guidance.

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 new guidance is effective on a prospective basis beginning on January 1, 2018 with early adoption permitted.

Fair Value Measurements and Short-term Investments

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements and Short-Term Investments

3. 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. The Company’s Level II assets consist of U.S. government bonds that are included in short-term investments.

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

		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

		December 31, 2016
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	89,626	$	—	$	—		$	89,626
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level II		77,295		8		(10	)		77,293
Total Securities		$	167,838	$	8	$	(10	)	$	167,836

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, 2017

Property Plant And Equipment [Abstract]

Property and Equipment

4. Property and Equipment

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

	December 31
	2017			2016
Machinery and equipment	$	7,162		$	5,973
Computer equipment and software		897			888
Furniture and fixtures		643			651
Leasehold improvements		701			578
Construction in progress		23			45
		9,426			8,135
Less: accumulated depreciation and amortization		(5,208	)		(3,743	)
	$	4,218		$	4,392

Depreciation and amortization expense was $1.5 million, $1.7 million and $1.2 million for the years ended December 31, 2017, 2016 and 2015, respectively.

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

5. 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 is accounted for as an indefinite-lived intangible asset until the underlying project is 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, 2017 was $0.1 million.

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

	December 31,
	2017		2016
Goodwill	$	949	$	949
In-process research and development		1,604		1,750

- Definition

The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final. May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.

+ References + Details

Accrued Liabilities

12 Months Ended

Dec. 31, 2017

Accrued Liabilities Current [Abstract]

Accrued Liabilities

6. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2017		2016
Research and clinical expenses	$	10,068	$	3,909
Payroll and related expenses		4,526		3,971
Legal and professional expenses		1,523		264
Property and equipment		—		331
Other accrued expenses		266		349
Total	$	16,383	$	8,824

Research and Collaboration Agreements	12 Months Ended
Research and Collaboration Agreements	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	7. Research and Collaboration Agreements AbbVie Ireland Unlimited Company In April 2016, the Company and AbbVie Ireland Unlimited Company (“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. AbbVie, at its sole discretion, may stop development of any CD71 PDC and terminate the CD71 Agreement if the Company does not meet certain preclinical research criteria by the applicable deadline. In such case, the Company and AbbVie may evaluate and approve an alternate CD71 PDC. If such alternate CD71 PDC is approved, then the Company and AbbVie will, in good faith, negotiate amendments to the timelines and, if necessary, the content in the research and development plan and budget and extensions to the deadlines to achieve defined success criteria. 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 and royalties on ex-US sales in the high teens to low twenties if the Company participates in the co-development of the CD71 Licensed Product subject to a reduction in such royalties 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 the associated sublicense fee), which the Company recognized as revenues during the period, from AbbVie for achieving certain milestones required to be met to begin GLP toxicology studies under the CD71 Agreement. 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 performance conditions under the CD71 Agreement. AbbVie has not selected the second target, but the performance conditions 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 deliverables 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 determined that the research, development and commercialization licenses for CD71 and discovery targets do not have standalone value without 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 also determined that the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee as a single unit of accounting has a standalone value 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 units of accounting: CD71 Agreement unit of accounting consisting of the CD71 Agreement research, development and commercialization license, related research service and participation in the joint research committee, and the Discovery Agreement unit of accounting consisting of the Discovery Agreement research, development and commercialization license, related research service and participation in the joint research committee. The upfront payments under the AbbVie Agreements are allocated between two units of accounting based on the estimated relative selling prices of each unit. In order to determine the best estimate of selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company recognizes the allocated amounts ratably over the estimated research service period of five years. The Company recognized revenue of $19.4 million and $3.2 million for the years ended December 31, 2017 and 2016, respectively, related to the AbbVie Agreements. As of December 31, 2017 and 2016, deferred revenue related to the CD71 Agreement unit of accounting was $12.0 million and $17.7 million, respectively, and deferred revenue related to the Discovery Agreement unit of accounting was $6.0 million and $8.9 million, respectively. 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 agreed to purchase 1,156,069 shares of the Company’s common stock, par value $0.00001 per share, 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. On the closing date, the Registration Rights Agreement (the “Registration Rights Agreement”) between the Company and Amgen went into effect. Pursuant to the Registration Rights Agreement, Amgen agreed not to dispose of any of the shares purchased during the six-month period following the closing date (the “lock-up period”) without the prior approval of a majority of the Company’s Board of Directors. 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 will be 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 only receive royalties in the low-double-digit to mid-teen percentage of commercial sales outside of the United States. 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 will 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 does not represent a deliverable of the agreement because it is a substantive option and was not issued at a significant or incremental discount. CytomX has 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. CytomX will be responsible, at its expense, for converting this program to a Probody T-cell engaging bispecific product, and thereafter, 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 605 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 significant deliverables 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 do not have stand-alone value from the research and development services and therefore those deliverables were combined into one unit of accounting. The Amgen Other Products will be accounted for as a separate unit of accounting from the EGFR Products as each has a standalone value to Amgen. 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 UCSB that is exclusively licensed to us 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. 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 our equity, was allocated between two units of accounting based on the estimated relative standalone selling price of each unit. To determine the best estimate of selling price, the Company used the discounted cash flow method by calculating risk-adjusted net present values of estimated cash flows. The Company will recognize the allocated amounts ratably over the estimated research service period. The Company estimated the fair value of the CytomX Product received from Amgen and the EGFR Products and the Amgen Products provided to Amgen based on significant unobservable inputs. Accordingly, they were considered to be level 3 fair value measurements. The significant inputs into these measurements were the Company’s evaluation of the probability of successful product development of 11%, remaining patent life at the time of product launch of 12 years, estimated projected future cash flows to be realized, and the Company’s estimated discount rate of 9%. The $10.7 million allocated to the CytomX Product was recorded to research and development expense because it has no alternative future use. The proceeds from the sale of our common stock, net of the $0.5 million premium, was recorded to equity. The estimated fair value of assets and services received approximates the total fair value of consideration given, resulting in no gain or loss recognized on the transaction. The Company recognized revenue of $1.3 million for the year ended December 31, 2017. As of December 31, 2017, deferred revenue relating to the EGFR Products and the Amgen Other Products was $45.3 million and $4.6 million, respectively. As of December 31, 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 were 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-digit to low double-digit percentage from potential future sales. The Company will also receive research and development service fees based on a prescribed full-time employee (“FTE”) rate that is capped. The BMS Agreement also required BMS to purchase the Company’s common stock upon an IPO if certain conditions were met. In connection with the IPO in October 2015, BMS purchased 833,333 shares of the Company’s common stock at the initial public offering price and on the same terms as other purchasers in the offering. The Company identified the following deliverables 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 does not have stand-alone value to BMS without the Company’s research services and expertise related to the development of the product candidates, and accordingly, it was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company received an upfront payment of $50.0 million from BMS in July 2014. The upfront payment was recorded as deferred revenue and being recognized on a ratable basis over the estimated performance period of ten years. The Company determined that the contingent payments under the BMS Agreement relating to development, sales milestone and royalties do not constitute substantive milestones and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events solely depends on BMS’s performance. In January 2016, BMS selected the third target pursuant to the BMS Agreement. Under the terms of the BMS Agreement, BMS paid the Company a $10.0 million payment. In December 2016, BMS selected the fourth and its final target pursuant to the BMS Agreement. Under the terms of the BMS Agreement, BMS paid the Company a $15.0 million payment. Both payments were recorded as deferred revenue and as a result of the fourth target selection, the performance period has been reduced from ten years to seven years and the deferred revenue is being recognized over this new performance period. 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, BMS received acceptance of the IND from the FDA for a CTLA-4-directed Probody therapeutic, which triggered a $10.0 million milestone payment to the Company. Both of these milestone payments were recognized as revenue in its entirety upon the achievement of the criteria necessary to earn the milestone payments. 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 will continue 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 was comprised of an upfront payment of $200.0 million and the Company will be 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; (iii) 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’ royalty obligation under the BMS Agreement. BMS’ 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 Company received an upfront payment from BMS under the Amendment of $200.0 million in May 2017. Upon receipt of the upfront payment from BMS, the Company made a payment of $10.0 million to the Regents of the University of California, acting through its Santa Barbara campus (“UCSB”), under the terms of our exclusive license agreement with UCSB. The upfront payment was recorded as deferred revenue and is being recognized on a ratable basis over the estimated performance period of eight years. In addition, the Company concluded the Amendment to be a modification of the BMS Agreement. As a result, the Company was recognizing the remaining deferred revenue balance relating to the upfront payment received under the BMS Agreement as of the Amendment Effective Date prospectively over the new estimated performance period of eight years. The Company determined that the contingent payments under the Amendment relating to development, sales milestone and royalties do not constitute substantive milestones and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments do not meet the definition of a substantive milestone because the achievement of these events solely depends on BMS’s performance. The Company recognized revenue of $36.5 million, $9.6 million and $5.9 million for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017 and 2016, deferred revenue relating to the BMS Agreement was $235.0 million and $60.9 million, respectively. The amount due from BMS under the BMS Agreement was $10.1 million and $2.2 million as of December 31, 2017 and 2016, respectively. ImmunoGen, Inc. In January 2014, the Company and ImmunoGen, Inc. (“ImmunoGen”) entered into the Research Collaboration Agreement (the “ImmunoGen Agreement”). The ImmunoGen Agreement provides 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 research license and to subsequently obtain exclusive, worldwide development and commercialization licenses to develop and commercialize such PDCs. In February 2017, ImmunoGen exercised its option to obtain a development and commercialization license for one of the two targets. ImmunoGen discontinued one of the two programs being developed under the ImmunoGen Agreement in July 2017 and substitution rights for this program terminated in February 2017. The Company recognized the remaining deferred revenue related to the discontinued program upon the termination of the program. ImmunoGen continues research work on the second collaboration target. Under the terms of the agreement, both the Company and ImmunoGen are required to perform research activities on behalf of the other party for no monetary consideration. 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. Each party may be liable to pay annual maintenance fees to the other party if the licensed product candidate covered under each development and commercialization license has not progressed to the clinical stage of development within six years of the exercise of the development and commercialization license. In consideration for the exclusive development and commercialization license that may be obtained by ImmunoGen, the Company is entitled to receive up to $30.0 million in development and regulatory milestone payments per the research program target, up to $50.0 million in sales milestone payments per target and royalties in the mid-single digits on the commercial sales of any resulting product. For the development and commercialization license that may be obtained by the Company, ImmunoGen is entitled to receive up to $60.0 million in development and regulatory milestone payments, up to $100.0 million in sales milestone payments and royalties in the mid to high single digits 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. The Company accounted for the ImmunoGen Agreement based on the fair value of the assets and services exchanged. The Company identified the following significant deliverables at the inception of the ImmunoGen 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, participation in the joint steering committee and the research services do not have stand-alone value from the development and commercialization license and therefore those deliverables were combined into one unit of accounting. The Company considered factors such the limited economic benefits to ImmunoGen if development and commercialization license is 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, of which $13.0 million, or $6.5 million per target, was allocated to the unit of accounting comprised of the research license, research services, participation in the joint research committee and the development and commercialization license, and $0.2 million was allocated to the future technological improvements. The Company recognized $6.6 million on the terminated program in March 2017. In December 2017, the Company entered into a license agreement with ImmunoGen (the “ImmunoGen Amendment”) pursuant to ImmunoGen’s exercise of its option to obtain a development and commercialization license for the second research program target under the ImmunoGen Agreement. The ImmunoGen Amendment extended the Company’s obligation to provide research services from January 8, 2018 to June 30, 2018. Upon the execution of the ImmunoGen Amendment, the final deliverable would now be the overall research services as the Company is obligated to provide research services to ImmunoGen through June 30, 2018. As a result, the Company made a cumulative catch-up adjustment and recognized $5.9 million of the total $6.6 million in deferred revenue through December 31, 2017. The estimated fair value of assets and services received was also $13.2 million during the year ended December 31, 2017, 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 was allocated to the research services, joint research committee participation and technology improvements, which will be expensed over the period of services to be provided. The Company recognized revenue of $12.5 million for the year ended December 31, 2017. No revenue was recognized for the years ended December 31, 2016 and December 31, 2015. As of December 31, 2017 and 2016, deferred revenue relating to the ImmunoGen Agreement was $0.7 million and $13.2 million, respectively. 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. In October 2017, the Company extended the research term of the agreement. Under the research collaboration agreement, the Company has the right to exercise an option, during the option period expiring on October 23, 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 will continue 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. 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. 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 of 2018. On March 6, 2018, Pfizer notified the Company that it was terminating the Pfizer Agreement. See Note 19, Subsequent Events for more details. The Company recognized revenue of $1.9 million, $2.2 million and $1.8 million for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017 and 2016, deferred revenue relating to the Pfizer Agreement was $1.6 million and $3.4 million, respectively. The amount due from Pfizer under the Pfizer Agreement was $0 million and $0.1 million as of December 31, 2017 and 2016, respectively.

License Agreement	12 Months Ended
License Agreement	Dec. 31, 2017
Research And Development [Abstract]
License Agreement	8. License Agreement The Company has an exclusive, worldwide license agreement (the “UCSB Agreement”) with the 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, 2017, 2016 and 2015, the Company incurred expenses of $13.5 million, $2.1 million and $0.3 million respectively, to UCSB under the provisions of the UCSB Agreement. 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.

Long-term Debt	12 Months Ended
Long-term Debt	Dec. 31, 2017
Debt Disclosure [Abstract]
Long-term Debt	9. Long-term Debt In May 2012, the Company entered into a Master Loan and Security Agreement (the “Debt Facility”). Under the terms of the agreement, an aggregate of $2.0 million could be drawn down during the initial basic loan term of 42 months. In January and December 2013, the Company amended the Debt Facility to borrow an additional $0.3 million and $3.0 million, respectively, with similar terms. Borrowings under the debt facility bore interest at 11.74% per annum. This debt was paid off during 2015. In connection with the execution and the amendment of the Debt Facility, the Company issued warrants to the lender to purchase an aggregate of 81,620 shares of the Company’s Series B-1 redeemable convertible preferred stock. The warrants were exercisable in cash at an exercise price of $3.084396 per share or through a cashless exercise provision. In connection with the consummation of the IPO in October 2015, all of the warrants were net exercised, resulting in issuance of an aggregate of 60,640 shares of the Company’s common stock. Upon issuance of the warrants, the Company recorded a preferred stock warrant liability based on its initial fair value estimated using the Black-Scholes model with an offset to debt discount. The debt discount was amortized to interest expense using the effective interest method over the term of the Debt Facility. The warrant liability was subject to remeasurement to fair value at each balance sheet date until the exercise date, and any change in fair value is recognized in other income (expense), net. The Company repaid and terminated the Debt Facility in September 2015.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2016

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

10. Commitments and Contingencies

Operating Lease

New Lease Agreement

On December 10, 2015, the Company entered into a lease (the “Lease”) with HCP Oyster Point III LLC (the “Landlord”) to lease approximately 76,173 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, 2016 and 2015.

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:
2018	$	4,724
2019		4,854
2020		4,990
2021		5,129
2022 and beyond		26,382
Total	$	46,079

Rent expense during the years ended December 31, 2017, 2016 and 2015 was $4.2 million, $1.8 million and $0.9 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.

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2017
Equity [Abstract]
Convertible Preferred Stock	11. Convertible Preferred Stock In December 2014, the Company granted a second tranche option (“Second Tranche Option”) to one of its investors to purchase 659,209 shares of its Series C redeemable convertible preferred stock upon the achievement of certain milestones. At initial recognition, the Company recorded the Second Tranche Option as a derivative liability on the balance sheet at its estimated fair value of $395,000. In May 2015, the Company achieved the relevant milestones and the investor exercised their right to purchase 659,209 shares of Series C convertible redeemable preferred stock for net proceeds of $3.5 million. Immediately prior to the closing of this tranche, the Company remeasured the preferred stock liability to its then fair value and recorded a loss from remeasurement of $1.1 million in other income (expense), net. The fair value of the preferred stock liability in the amount of $1.5 million was reclassified to redeemable convertible preferred stock. In connection with the consummation of the IPO in October 2015, all outstanding shares of Series A-1, Series A-2, Series B-1, Series B-2, Series C and Series D were converted into 27,135,453 shares of common stock on a one-for-one basis. As such, no convertible preferred stock shares were outstanding as of December 31, 2017 and 2016.

Common Stock

12 Months Ended

Dec. 31, 2017

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, 2017 and 2016, 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,
	2017		2016
Options issued and outstanding		6,503,458		6,158,746
Shares available for future stock option grants		2,324,793		2,493,188
Total		8,828,251		8,651,934

Stock-based Compensation

12 Months Ended

Dec. 31, 2017

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, 2017, 2,324,793 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, 2014		1,896,617			2,147,872		$	1.197
Options authorized		3,801,597			—			—
Options granted		(3,309,708	)		3,309,708			5.174
Options exercised		—			(173,929	)		1.507
Options forfeited		12,900			(12,900	)		1.405
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		7.2	$	84,243
Options Exercisable—December 31, 2017					3,727,754			5.509		6.2	$	58,159
Options vested and expected to vest—December 31, 2017					6,503,458			8.157		7.2	$	84,243

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, 2017, 2016 and 2015, respectively.

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2017, 2016 and 2015 was $10.5 million, $4.6 million and $2.3 million, respectively.

The options granted in the years ended December 31, 2017, 2016 and 2016 had a weighted-average per share grant-date fair value of $8.207, $8.936, and $7.169, respectively. At December 31, 2017, the unrecognized compensation expense with respect to options granted to employees was $22.3 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. We issued 67,746 shares and 31,564 shares of common stock under the ESPP in 2017 and 2016, respectively.

Shares available for future purchase under the ESPP were 980,389 at December 31, 2017. The compensation expense related to the ESPP was $247,000, $145,000 and $0 for the years ended December 31, 2017, 2016 and 2015, respectively. As of December 31, 2017 and 2016, there was $123,000 and $113,000, respectively, of unrecognized compensation cost related to the ESPP, which we expect 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,
	2017		2016		2015
Research and development	$	5,161	$	4,925	$	1,972
General and administrative		6,126		5,170		2,014
Total stock-based compensation expense	$	11,287	$	10,095	$	3,986

Stock-based compensation expense for employees was $11.0 million, $9.4 million and $3.2 million for the years ended December 31, 2017, 2016 and 2015, 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 $0.3 million, $0.9 million and $0.8 million for the years ended December 31, 2017, 2016 and 2015, 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,
	2017	2016	2015	2017	2016
Expected volatility	69.1%-72.4%	76.4% – 83.5%	62.9% – 68.9%	52.3%-63.8%	50.4% – 75.6%
Risk-free interest rate	1.7%-2.2%	1.2% – 2.1%	1.4% – 1.9%	1.1%-1.5%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %	— %
Expected term (in years)	4.9-5.3	5.3 – 5.9	5.2 – 7.2	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.

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2017
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	14. Related Party Transactions In 2015, certain employees of Third Rock Ventures, a greater than 10% stockholder of the Company, provided consulting services to the Company. One of the Company’s board members is affiliated with Third Rock Ventures. General and administrative expenses for consulting services and board service fees incurred were $35,000, $48,000 and $33,000 for the years ended December 31, 2017, 2016 and 2015, 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 and $1.8 million for the years ended December 31, 2016 and 2015, respectively. 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.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

15. Income Taxes

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

	Years Ended December 31,
	2017			2016			2015
Current:
Federal	$	—		$	—		$	—
State		1			1			2
Total current		1			1			2
Deferred:
Federal		(514	)		(20	)		8
State		—			—			—
Total deferred		(514	)		(20	)		8
Provision for (benefit from) income taxes	$	(513	)	$	(19	)	$	10

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

	Years Ended December 31,
	2017			2016			2015
U.S. federal taxes at statutory rate		34.0	%		34.0	%		34.0	%
State tax, net of federal benefit		7.6	%		0.8	%		0.8	%
Stock compensation		2.3	%		(0.6	)%		(1.1	)%
Tax attributes subject to 382 limitation		27.5	%		0.0	%		(35.4	)%
Tax credits		2.7	%		2.2	%		0.8	%
Change in valuation allowance		(9.3	)%		(35.6	)%		2.7	%
Change in deferreds due to rate change		(58.6	)%		0.0	%		0.0	%
Other		(5.0	)%		(0.8	)%		(1.8	)%
Total		1.2	%		0.0	%		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,
	2017			2016			2015
Net operating loss carryforwards	$	24,682		$	24,528		$	5,688
Research and development credits		5,757			2,683			1,337
Intangible—in-process R&D		—			81			88
Deferred revenue		15,631			13,857			16,182
Accruals and deferred rent		1,256			1,335			998
Stock-based compensation		3,831			3,963			1,125
Other		32			1			26
Total gross deferred income tax assets		51,189			46,448			25,444
Less: valuation allowance		(50,791	)		(46,137	)		(25,043	)
Deferred tax assets, net of valuation allowance		398			311			401
Fixed assets		(282	)		(229	)		(313	)
In-process R&D		—			(595	)		(595	)
Intangible assets		(116	)		—			—
Deferred tax liabilities		(398	)		(824	)		(908	)
Net deferred income tax liabilities	$	—		$	(513	)	$	(507	)

A valuation allowance has been established for the portion of deferred assets for which realization is not probable. The net change in the total valuation allowance for the year ended December 31, 2017 and 2016 was an increase of $4.7 million and an increase of $21.1 million, respectively and for the year ended December 31, 2015 was a decrease of $1.0 million.

The Company had net operating loss carryforwards for federal and state income tax purposes of approximately $105.6 million and $58.6 million, respectively, as of December 31, 2017 available to reduce future income subject to income taxes. The federal and state net operating loss carryforwards will begin to expire in 2030 if not utilized.

The Company also has federal and state research and development tax credits carryforwards of $4.6 million and $3.9 million, respectively, as of December 31, 2017 available to reduce future income taxes. The federal research and development tax credits will begin to expire in 2030 if not utilized. The state research and development tax credits have no expiration date.

On December 22, 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. The Company has calculated its best estimate of the impact of the Tax Act in accordance with our understanding of the Tax Act and guidance available as of the date of this filing. The tax rate decrease resulted in a reduction of $25.7 million in our deferred tax assets, and a corresponding decrease of the same amount in the valuation allowance against these deferred tax assets, as substantially all of our U.S. deferred tax assets, net of deferred tax liabilities, are subject to a full valuation allowance. The deferred tax asset remeasurement is provisional because the Company continues to evaluate the impact of various domestic provisions of the Act as well as the impact of additional guidance that may be provided.

Due to the adoption of ASU 2016-09 in 2017, the Company recorded an immaterial increase in the deferred tax assets for previously unrecognized excess tax benefits that existed as of December 31, 2016, and a corresponding increase in the valuation allowance against these deferred tax assets. In addition, all excess tax benefits and deficiencies are recognized as income tax expense and will result in increased volatility in the Company’s income tax.

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. None of the identified ownership changes resulted in Section 382 limitations with material restrictions such that all of the tax attributes become available for use prior to expiration of their respective carryforward periods. Accordingly, none of the tax attributes have been reduced. 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 $4.3 million and $1.2 million of unrecognized tax benefits as of December 31, 2017 and December 31, 2016, respectively, none of which would affect the Company’s effective tax rate if recognized, due to the Company’s valuation allowance.

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

	Year Ended December 31,
	2017		2016		2015
Balance at the beginning of the year	$	1,182	$	666	$	3,019
Additions based on tax positions related to current year		521		23		(2,312	)
Adjustment based on submitted prior year tax returns		2,617		493		(41	)
Balance at end of the year	$	4,320	$	1,182	$	666

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 at December 31, 2017, 2016 and 2015.

The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2017 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, 2017, the Company is not under examination by the Internal Revenue Service or any state or foreign tax jurisdiction.

Net Loss Per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

17. Net Loss Per Share Attributable to Common Stockholders

The following weighted-average outstanding shares of potentially dilutive securities were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods presented, because including them would have been anti-dilutive:

	Year Ended December 31,
	2017		2016		2015
Redeemable convertible preferred stock (on an as-converted basis)		—		—		17,507,788
Convertible preferred stock (on an as-converted basis)		—		—		192,473
Options to purchase common stock		6,891,123		6,086,939		3,865,842
Convertible preferred stock warrants		—		—		64,178
Total		6,891,123		6,086,939		21,630,281

A reconciliation of the numerator and denominator used in the calculation of the basic and diluted net loss per share attributable to common stockholders is as follows (in thousands except share and per share amounts):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(43,099	)	$	(58,900	)	$	(35,374	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			—			(6,705	)
Net loss attributable to common stockholders		(43,099	)		(58,900	)		(42,079	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		37,166,830			36,234,732			8,595,247
Net loss per share attributable to common stockholders, basic and diluted	$	(1.16	)	$	(1.63	)	$	(4.90	)

Supplementary Data - Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Supplementary Data – Quarterly Financial Data (Unaudited)

18. Supplementary Data – Quarterly Financial Data (Unaudited)

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2017 and 2016:

	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	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2016			September 30, 2016			June 30, 2016			March 31, 2016
Revenue	$	6,272		$	3,454		$	3,094		$	2,223
Net loss	$	(14,033	)	$	(14,662	)	$	(14,176	)	$	(16,029	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.39	)	$	(0.40	)	$	(0.39	)	$	(0.44	)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	19. Subsequent Event On March 6, 2018, Pfizer notified the Company that it was terminating the Research Collaboration, Option and License Agreement. Such termination will become effective 60 days after the notification. As a result of such termination, the Company is no longer eligible to receive any further proceeds from license options, milestones, royalties or research and development fees.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

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
	For the Year Ended December 31,						December 31,
	2017		2016		2015		2017		2016
AbbVie Ireland Unlimited Company	$	19,434	$	3,268	$	—	**		$	—
Bristol-Myers Squibb Company		36,492		9,577		5,941		10,126		2,159
ImmunoGen, Inc.		12,503		—		—		—		—
Pfizer Inc.	*			2,198		1,771	**		**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	68,429	$	15,043	$	7,712	$	10,126	$	2,159

* 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.

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

Accrued Research and Development Costs

Convertible Preferred Stock Warrant Liability

Convertible Preferred Stock Liability

Comprehensive Income (Loss)

Revenue Recognition

Stock-Based Compensation

Income Taxes

Net Loss per Share Attributable to Common Stockholders

Reclassifications

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

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
	For the Year Ended December 31,						December 31,
	2017		2016		2015		2017		2016
AbbVie Ireland Unlimited Company	$	19,434	$	3,268	$	—	**		$	—
Bristol-Myers Squibb Company		36,492		9,577		5,941		10,126		2,159
ImmunoGen, Inc.		12,503		—		—		—		—
Pfizer Inc.	*			2,198		1,771	**		**
Total revenue from customers who represent 10% or more of the Company's total revenue	$	68,429	$	15,043	$	7,712	$	10,126	$	2,159

* 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 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

Fair Value Measurements and Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis

		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

		December 31, 2016
	Valuation Hierarchy	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Assets
Money market funds	Level I	$	89,626	$	—	$	—		$	89,626
Restricted cash (money market funds)	Level I		917		—		—			917
U.S. Government bonds	Level II		77,295		8		(10	)		77,293
Total Securities		$	167,838	$	8	$	(10	)	$	167,836

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

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

	December 31
	2017			2016
Machinery and equipment	$	7,162		$	5,973
Computer equipment and software		897			888
Furniture and fixtures		643			651
Leasehold improvements		701			578
Construction in progress		23			45
		9,426			8,135
Less: accumulated depreciation and amortization		(5,208	)		(3,743	)
	$	4,218		$	4,392

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2017

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

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

	December 31,
	2017		2016
Goodwill	$	949	$	949
In-process research and development		1,604		1,750

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2017

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2017		2016
Research and clinical expenses	$	10,068	$	3,909
Payroll and related expenses		4,526		3,971
Legal and professional expenses		1,523		264
Property and equipment		—		331
Other accrued expenses		266		349
Total	$	16,383	$	8,824

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2016

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:
2018	$	4,724
2019		4,854
2020		4,990
2021		5,129
2022 and beyond		26,382
Total	$	46,079

Common Stock (Tables)

12 Months Ended

Dec. 31, 2017

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,
	2017		2016
Options issued and outstanding		6,503,458		6,158,746
Shares available for future stock option grants		2,324,793		2,493,188
Total		8,828,251		8,651,934

- 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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+ Details

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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, 2014		1,896,617			2,147,872		$	1.197
Options authorized		3,801,597			—			—
Options granted		(3,309,708	)		3,309,708			5.174
Options exercised		—			(173,929	)		1.507
Options forfeited		12,900			(12,900	)		1.405
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		7.2	$	84,243
Options Exercisable—December 31, 2017					3,727,754			5.509		6.2	$	58,159
Options vested and expected to vest—December 31, 2017					6,503,458			8.157		7.2	$	84,243

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,
	2017		2016		2015
Research and development	$	5,161	$	4,925	$	1,972
General and administrative		6,126		5,170		2,014
Total stock-based compensation expense	$	11,287	$	10,095	$	3,986

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,
	2017	2016	2015	2017	2016
Expected volatility	69.1%-72.4%	76.4% – 83.5%	62.9% – 68.9%	52.3%-63.8%	50.4% – 75.6%
Risk-free interest rate	1.7%-2.2%	1.2% – 2.1%	1.4% – 1.9%	1.1%-1.5%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %	— %
Expected term (in years)	4.9-5.3	5.3 – 5.9	5.2 – 7.2	0.5	0.5

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

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

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

	Years Ended December 31,
	2017			2016			2015
Current:
Federal	$	—		$	—		$	—
State		1			1			2
Total current		1			1			2
Deferred:
Federal		(514	)		(20	)		8
State		—			—			—
Total deferred		(514	)		(20	)		8
Provision for (benefit from) income taxes	$	(513	)	$	(19	)	$	10

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,
	2017			2016			2015
U.S. federal taxes at statutory rate		34.0	%		34.0	%		34.0	%
State tax, net of federal benefit		7.6	%		0.8	%		0.8	%
Stock compensation		2.3	%		(0.6	)%		(1.1	)%
Tax attributes subject to 382 limitation		27.5	%		0.0	%		(35.4	)%
Tax credits		2.7	%		2.2	%		0.8	%
Change in valuation allowance		(9.3	)%		(35.6	)%		2.7	%
Change in deferreds due to rate change		(58.6	)%		0.0	%		0.0	%
Other		(5.0	)%		(0.8	)%		(1.8	)%
Total		1.2	%		0.0	%		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,
	2017			2016			2015
Net operating loss carryforwards	$	24,682		$	24,528		$	5,688
Research and development credits		5,757			2,683			1,337
Intangible—in-process R&D		—			81			88
Deferred revenue		15,631			13,857			16,182
Accruals and deferred rent		1,256			1,335			998
Stock-based compensation		3,831			3,963			1,125
Other		32			1			26
Total gross deferred income tax assets		51,189			46,448			25,444
Less: valuation allowance		(50,791	)		(46,137	)		(25,043	)
Deferred tax assets, net of valuation allowance		398			311			401
Fixed assets		(282	)		(229	)		(313	)
In-process R&D		—			(595	)		(595	)
Intangible assets		(116	)		—			—
Deferred tax liabilities		(398	)		(824	)		(908	)
Net deferred income tax liabilities	$	—		$	(513	)	$	(507	)

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,
	2017		2016		2015
Balance at the beginning of the year	$	1,182	$	666	$	3,019
Additions based on tax positions related to current year		521		23		(2,312	)
Adjustment based on submitted prior year tax returns		2,617		493		(41	)
Balance at end of the year	$	4,320	$	1,182	$	666

Net Loss Per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

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

	Year Ended December 31,
	2017		2016		2015
Redeemable convertible preferred stock (on an as-converted basis)		—		—		17,507,788
Convertible preferred stock (on an as-converted basis)		—		—		192,473
Options to purchase common stock		6,891,123		6,086,939		3,865,842
Convertible preferred stock warrants		—		—		64,178
Total		6,891,123		6,086,939		21,630,281

Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(43,099	)	$	(58,900	)	$	(35,374	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			—			(6,705	)
Net loss attributable to common stockholders		(43,099	)		(58,900	)		(42,079	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		37,166,830			36,234,732			8,595,247
Net loss per share attributable to common stockholders, basic and diluted	$	(1.16	)	$	(1.63	)	$	(4.90	)

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

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Unaudited Financial Information

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2017 and 2016:

	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	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2016			September 30, 2016			June 30, 2016			March 31, 2016
Revenue	$	6,272		$	3,454		$	3,094		$	2,223
Net loss	$	(14,033	)	$	(14,662	)	$	(14,176	)	$	(16,029	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.39	)	$	(0.40	)	$	(0.39	)	$	(0.44	)

Description of the Business - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Oct. 04, 2017	Oct. 14, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses				$ 81,777
Common stock, par value		$ 0.00001	$ 0.00001		$ 0.00001
Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of common stock, shares			11,000	7,666,667
Common Stock \| Amgen Inc
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Common stock, aggregate cash purchase price	$ 20
Initial Public Offering \| Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Price per share		$ 12.00
Issuance of common stock, shares		7,666,667
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses		$ 81,800
Underwriters \| Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of common stock, shares		1,000,000
Private Placement \| Amgen Inc
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Common stock, par value	$ 0.00001
Private Placement \| Common Stock \| Amgen Inc
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Price per share	$ 17.30
Issuance of common stock, shares	1,156,069
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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Revenue	$ 27,074	$ 24,144	$ 8,752	$ 11,653	$ 6,272	$ 3,454	$ 3,094	$ 2,223	$ 71,623	$ 15,043	$ 7,712
Accounts Receivable, net	10,139				2,159				10,139	2,159
Major Customer
Concentration Risk [Line Items]
Revenue									68,429	15,043	7,712
Accounts Receivable, net	10,126				2,159				10,126	2,159
Major Customer \| AbbVie Ireland Unlimited Company
Concentration Risk [Line Items]
Revenue									19,434	3,268
Major Customer \| Bristol Myers Squibb Company
Concentration Risk [Line Items]
Revenue									36,492	9,577	5,941
Accounts Receivable, net	$ 10,126				$ 2,159				10,126	2,159
Major Customer \| Immuno Gen Inc
Concentration Risk [Line Items]
Revenue									$ 12,503
Major Customer \| Pfizer Inc
Concentration Risk [Line Items]
Revenue										$ 2,198	$ 1,771

Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Activity Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2015 shares
Summary Of Significant Accounting Policies [Line Items]
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
Convertible preferred stock, shares issued \| shares				27,135,453
Interest or penalties charged in relation to unrecognized tax benefits	0
Deferred revenue	189,913,000	$ 43,317,000	$ (6,130,000)
Adjustments Due to ASC 606 \| Minimum
Summary Of Significant Accounting Policies [Line Items]
Change in accumulated deficit	(6,000,000)
Deferred revenue	6,000,000
Adjustments Due to ASC 606 \| Maximum
Summary Of Significant Accounting Policies [Line Items]
Change in accumulated deficit	(10,000,000)
Deferred revenue	$ 10,000,000
Initial Public Offering \| Common Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock, shares issued \| shares				27,135,453

Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
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

Fair Value Measurements and Investments - Schedule of Short-Term Investments Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 361,986	$ 167,838
Gross Unrealized Holding Gains		8
Gross Unrealized Holding Losses	(67)	(10)
Aggregate Fair Value	361,919	167,836
Level I \| Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	164,440	89,626
Aggregate Fair Value	164,440	89,626
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	196,629
Gross Unrealized Holding Losses	(67)
Aggregate Fair Value	$ 196,562
Level II \| U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost		77,295
Gross Unrealized Holding Gains		8
Gross Unrealized Holding Losses		(10)
Aggregate Fair Value		$ 77,293

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Total property and equipment	$ 9,426	$ 8,135
Less: accumulated depreciation and amortization	(5,208)	(3,743)
Property and equipment, net	4,218	4,392
Machinery and equipment
Property Plant And Equipment [Line Items]
Total property and equipment	7,162	5,973
Computer equipment and software
Property Plant And Equipment [Line Items]
Total property and equipment	897	888
Furniture and fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	643	651
Leasehold improvements
Property Plant And Equipment [Line Items]
Total property and equipment	701	578
Construction in progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 23	$ 45

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

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill	$ 949	$ 949
In-process research and development	$ 1,604	$ 1,750

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Research and clinical expenses	$ 10,068	$ 3,909
Payroll and related expenses	4,526	3,971
Legal and professional expenses	1,523	264
Property and equipment		331
Other accrued expenses	266	349
Total	$ 16,383	$ 8,824

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details) - AbbVie Ireland Unlimited Company	1 Months Ended	12 Months Ended
	Apr. 30, 2016 USD ($) Agreement Target AccountingUnit	Dec. 31, 2017 USD ($) Target	Dec. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of accounting units \| AccountingUnit	2
Estimated research service period		5 years
Revenue recognized from collaborative arrangement		$ 19,400,000	$ 3,200,000
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%
CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received	$ 14,000,000
Deferred revenue		$ 12,000,000	17,700,000
CD71 Agreement \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	20,000,000
CD71 Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones	470,000,000
Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets selected \| Target		1
Deferred revenue		$ 6,000,000	$ 8,900,000
Discovery Agreement \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 10,000,000
Discovery Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target	2
Discovery Agreement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 275,000,000

Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)		1 Months Ended	12 Months Ended
	Sep. 29, 2017 USD ($) Target	Oct. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Oct. 14, 2015 $ / shares
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, par value \| $ / shares			$ 0.00001	$ 0.00001	$ 0.00001
Common stock, value of shares issued in connection with agreement			$ 19,957,000
Liabilities Current			61,147,000	$ 35,767,000
Probability for successful product development, percentage	11.00%
Product Liability Contingency, Accrual, Discount Rate	9.00%
Patents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Finite-Lived Intangible Asset, Useful Life	12 years
Collaboration and License Agreement \| Amgen Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issuable under agreement \| shares		1,156,069
Common stock, shares issuable under agreement, price per share \| $ / shares		$ 17.30
Common stock, par value \| $ / shares		$ 0.00001
Common stock, value of shares issued in connection with agreement		$ 20,000
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 \| Target	3
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 equity	500,000
Estimated fair value of products	10,700,000
Non-cash acquisition of in-process research and development expense			10,700,000
Revenue recognized from collaborative arrangement			1,300,000
Amount due from agreement			0
Collaboration and License Agreement \| Amgen Inc \| EGFR Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			45,300,000
Collaboration and License Agreement \| Amgen Inc \| Amgen Other Products
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue			$ 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
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%
Collaboration and License Agreement \| Amgen Inc \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 40,000
Sublicense Agreement \| Amgen Inc \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment percentage	15.00%
Liabilities Current	$ 2,100,000

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)			1 Months Ended		12 Months Ended
	Apr. 25, 2017 USD ($) Target SaleIndicator	Jul. 07, 2014 USD ($) Target Term ResearchTarget SaleIndicator	May 31, 2017 USD ($)	Oct. 31, 2015 shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Target	Dec. 31, 2015 USD ($)	Jan. 31, 2016 USD ($)
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						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.
Extension of research term for each collaboration target		1 year
Contingent milestone payments receivable		$ 1,217,000,000			$ 15,000,000			$ 10,000,000
Estimated performance period of agreement	8 years				10 years
Revised estimated performance period of agreement						7 years
Revenue recognized from collaborative arrangement					$ 36,500,000	$ 9,600,000	$ 5,900,000
Deferred revenue						235,000,000	60,900,000
Amount due from agreement						10,100,000	$ 2,200,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| UC Regents
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone and minimum annual royalty provision paid			$ 10,000,000
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 \| Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		25,000,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achievement Of Development Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		114,000,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achievement Of Development Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 456,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 124,000,000
Number of sales indicators \| SaleIndicator		3
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achieving First Commercial Sale If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 496,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achieving Sales Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 60,000,000
Collaborative Arrangement \| Bristol Myers Squibb Company \| Achieving Sales Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 240,000,000
Number of research targets selected \| ResearchTarget		4
Collaborative Arrangement \| Bristol Myers Squibb Company \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 200,000,000	$ 50,000,000	$ 200,000,000
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						$ 3,586,000,000
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
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of sales indicators \| SaleIndicator	3
Aggregate future contingent milestone payment	$ 124,000,000
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
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]
Number of research targets selected \| Target	8
Aggregate future contingent milestone payment	$ 928,000,000
Collaborative Arrangement \| Maximum \| Bristol Myers Squibb Company \| Achieving Eight Targets Selected for the First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets selected \| Target	8
Aggregate future contingent milestone payment	$ 992,000,000
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]
Number of research targets selected \| Target	8
Aggregate future contingent milestone payment	$ 480,000,000
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]
Number of research targets selected \| Target	8
Aggregate future contingent milestone payment	$ 450,000,000
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
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]
Number of research targets selected \| Target	8
Number of sales indicators \| SaleIndicator	3
Aggregate future contingent milestone payment	$ 496,000,000
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]
Number of research targets selected \| Target	8
Aggregate future contingent milestone payment	$ 240,000,000
Bristol Myers Squibb Agreement \| Initial Public Offering
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock, shares issued in connection with agreement \| shares				833,333
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) - Collaborative Arrangement - Immuno Gen Inc - USD ($)	1 Months Ended			12 Months Ended
	Aug. 31, 2017	Mar. 31, 2017	Jan. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestones payments made during period	$ 1,000,000
Estimated total fair value consideration recorded as deferred revenue			$ 13,200,000	$ 700,000	$ 13,200,000
Revenue recognized from collaborative arrangement				12,500,000	$ 0	$ 0
Estimated fair value of assets and services			13,200,000
Licenses Received
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of assets and services			12,700,000
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
Termination Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement		$ 6,600,000
Revenue recognition cumulative catch-up adjustment				$ 5,900,000
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
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment			0
Research License, Research Services, Joint Research Committee, Commercialization License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			13,000,000
Estimated total fair value consideration recorded as deferred revenue, per target			6,500,000
Technology Improvements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue			$ 200,000

Research and Collaboration Agreements - Pfizer Inc - Additional Information (Details) - Collaborative Arrangement - Pfizer Inc $ in Millions	1 Months Ended	12 Months Ended
	May 31, 2013 Target	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target	2
Nominated number of additional research targets \| Target	2
Revenue recognized from collaborative arrangement		$ 1.9	$ 2.2	$ 1.8
Deferred revenue		1.6	3.4
Amount due from agreement		$ 0.0	$ 0.1

License Agreement - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	38,478,560	36,490,169
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	$ 13,500,000	$ 2,100,000	$ 300,000

Long-term Debt - Additional Information (Details) - USD ($)	1 Months Ended
	Oct. 31, 2015	May 31, 2012	Dec. 31, 2017	Dec. 31, 2013	Jan. 31, 2013
Debt Instrument [Line Items]
Warrants exercise price			$ 3.084396
Common Stock \| Initial Public Offering
Debt Instrument [Line Items]
Convertible preferred stock, shares issued	60,640
Series B-1 Redeemable Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants issued to purchase stock			81,620
Master Loan and Security Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 2,000,000
Line of credit facility, initial basic loan term		42 months
Line of credit facility, additional borrowing capacity				$ 3,000,000	$ 300,000
Debt facility interest rate		11.74%

Commitments and Contingencies - Additional Information (Details)		12 Months Ended
	Dec. 10, 2015 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Operating Leased Assets [Line Items]
Rent expense		$ 4,200,000	$ 1,800,000	$ 900,000
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	76,173
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, 2017 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2018	$ 4,724
2019	4,854
2020	4,990
2021	5,129
2022 and beyond	26,382
Total	$ 46,079

Convertible Preferred Stock - Additional information (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Oct. 31, 2015	Dec. 31, 2014
Class Of Stock [Line Items]
Convertible preferred stock, shares issued				27,135,453
Convertible preferred stock, conversion basis		one-for-one
Shares Outstanding		0	0
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares Issued	659,209
Convertible preferred stock liability					$ 395,000
Proceeds from issuance of redeemable preferred stock	$ 3,500,000
Fair value of preferred stock liability reclassified to redeemable convertible preferred stock	1,500,000
Series C Redeemable Convertible Preferred Stock \| Other Income (Expense), Net
Class Of Stock [Line Items]
Gain (loss) on re-measurement of preferred stock liability	$ 1,100,000
Series C Redeemable Convertible Preferred Stock \| Second Tranche Option
Class Of Stock [Line Items]
Preferred stock, shares granted					659,209

Common Stock - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	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, 2017	Dec. 31, 2016
Equity [Abstract]
Options issued and outstanding	6,503,458	6,158,746
Shares available for future stock option grants	2,324,793	2,493,188
Total	8,828,251	8,651,934

Stock-based Compensation - Additional Information (Details) - USD ($)	12 Months Ended
	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	8,828,251	8,651,934
Stock option expiration period	10 years
Aggregate intrinsic value of stock options exercised	$ 10,500,000	$ 4,600,000	$ 2,300,000
Weighted average grant date fair value, options granted	$ 8.207	$ 8.936	$ 7.169
Unrecognized compensation expense, options granted to employees	$ 22,300,000
Number of shares issued during period	67,746	31,564
Stock-based compensation expense	$ 11,287,000	$ 10,095,000	$ 3,986,000
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	980,389
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	$ 247,000	145,000	0
Unrecognized stock-based compensation cost	$ 123,000	113,000
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	$ 11,000,000	9,400,000	3,200,000
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 300,000	$ 900,000	$ 800,000
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	2,324,793		2,444,735
Aggregate intrinsic value of stock options exercised		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, 2017	Dec. 31, 2016	Dec. 31, 2015
Options Available for Grant
Balances, beginning of the period	2,493,188
Balances, end of the period	2,324,793	2,493,188
Number of Options
Balances, beginning of the period	6,158,746	5,270,751	2,147,872
Options granted	2,138,620	1,356,546	3,309,708
Options exercised	(764,576)	(414,396)	(173,929)
Options forfeited	(1,029,332)	(54,155)	(12,900)
Balances, end of the period	6,503,458	6,158,746	5,270,751
Options Exercisable, end of the period	3,727,754
Options vested and expected to vest, end of the period	6,503,458
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 3.694	$ 1.197	$ 1.197
Options granted	13.566	13.234	5.174
Options exercised	4.140	1.549	1.507
Options forfeited	9.118	4.578	1.405
Balances, end of the period	8.157	$ 3.694	$ 1.197
Options Exercisable, end of the period	5.509
Options vested and expected to vest, end of the period	$ 8.157
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	7 years 2 months 12 days
Options Exercisable, end of the period	6 years 2 months 12 days
Options vested and expected to vest, end of the period	7 years 2 months 12 days
Aggregate Intrinsic Value
Balances, end of the period	$ 84,243
Options Exercisable—December 31, 2017	58,159
Options vested and expected to vest—December 31, 2017	$ 84,243
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	2,493,188	2,401,406	1,896,617
Options authorized	1,459,606	1,441,328	3,801,597
Options granted	(2,138,620)	(1,367,546)	(3,309,708)
Options forfeited	510,619	18,000	12,900
Balances, end of the period	2,324,793	2,493,188	2,401,406

Stock-based Compensation - Total Stock-based Compensation Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-based compensation expense:
Stock-based compensation expense	$ 11,287	$ 10,095	$ 3,986
Research and development
Stock-based compensation expense:
Stock-based compensation expense	5,161	4,925	1,972
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 6,126	$ 5,170	$ 2,014

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	69.10%	76.40%	62.90%
Expected volatility, maximum	72.40%	83.50%	68.90%
Risk-free interest rate, minimum	1.70%	1.20%	1.40%
Risk-free interest rate, maximum	2.20%	2.10%	1.90%
Employee Stock Option \| Minimum
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 2 months 12 days
Employee Stock Option \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 10 months 24 days	7 years 2 months 12 days
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	52.30%	50.40%
Expected volatility, maximum	63.80%	75.60%
Risk-free interest rate, minimum	1.10%	0.50%
Risk-free interest rate, maximum	1.50%	0.60%
Expected term (in years)	6 months	6 months

Related Party Transactions - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) BoardMember	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangement \| Pfizer Inc
Related Party Transaction [Line Items]
Threshold limit to be qualified as related party	10.00%
Revenue recognized	$ 1,900,000	$ 2,200,000	$ 1,800,000
Deferred revenue	1,600,000	3,400,000
Amount due from agreement	$ 0	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
General and administrative expense from transactions with related party	$ 35,000	48,000	$ 33,000
Accounts payable, related parties	$ 0	$ 12,000

Income Taxes - Schedule of Components of Provision (Benefit from) for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
State	$ 1	$ 1	$ 2
Total current	1	1	2
Deferred:
Federal	(514)	(20)	8
Total deferred	(514)	(20)	8
Provision for (benefit from) income taxes	$ (513)	$ (19)	$ 10

Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation [Abstract]
U.S. federal taxes at statutory rate	34.00%	34.00%	34.00%
State tax, net of federal benefit	7.60%	0.80%	0.80%
Stock compensation	2.30%	(0.60%)	(1.10%)
Tax attributes subject to 382 limitation	27.50%	0.00%	(35.40%)
Tax credits	2.70%	2.20%	0.80%
Change in valuation allowance	(9.30%)	(35.60%)	2.70%
Change in deferreds due to rate change	(58.60%)	0.00%	0.00%
Other	(5.00%)	(0.80%)	(1.80%)
Total	1.20%	0.00%	0.00%

Income Taxes - Schedule of Deferred Income Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 24,682	$ 24,528	$ 5,688
Research and development credits	5,757	2,683	1,337
Intangible—in-process R&D		81	88
Deferred revenue	15,631	13,857	16,182
Accruals and deferred rent	1,256	1,335	998
Stock-based compensation	3,831	3,963	1,125
Other	32	1	26
Total gross deferred income tax assets	51,189	46,448	25,444
Less: valuation allowance	(50,791)	(46,137)	(25,043)
Deferred tax assets, net of valuation allowance	398	311	401
Fixed assets	(282)	(229)	(313)
In-process R&D		(595)	(595)
Intangible assets	(116)
Deferred tax liabilities	$ (398)	(824)	(908)
Net deferred income tax liabilities		$ (513)	$ (507)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Deferred tax asset, net change in total valuation allowance increase (decrease) during period		$ 4,700,000	$ 21,100,000	$ (1,000,000)
Federal statutory income tax rate		34.00%	34.00%	34.00%
Change in deferred tax asset due to tax cuts and jobs act of 2017		$ 25,700,000
Unrecognized tax benefits		4,320,000	$ 1,182,000	$ 666,000	$ 3,019,000
Unrecognized tax benefits which would affect company's effective tax rate due to valuation allowance		0
Interest and penalties accrued		$ 0	$ 0	$ 0
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%
Scenario Forecast
Income Tax Contingency [Line Items]
Federal statutory income tax rate	21.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 105,600,000
Operating loss carryforwards expiration year		2030
Federal \| 2010
Income Tax Contingency [Line Items]
Open tax year		2010
Federal \| 2014
Income Tax Contingency [Line Items]
Open tax year		2017
Federal \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards		$ 4,600,000
Tax credit carryforward, expiration		2030
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 58,600,000
Operating loss carryforwards expiration year		2030
State \| 2010
Income Tax Contingency [Line Items]
Open tax year		2010
State \| 2014
Income Tax Contingency [Line Items]
Open tax year		2017
State \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards		$ 3,900,000

Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at the beginning of the year	$ 1,182	$ 666	$ 3,019
Additions based on tax positions related to current year	521	23	(2,312)
Adjustment based on submitted prior year tax returns	2,617	493	(41)
Balance at end of the year	$ 4,320	$ 1,182	$ 666

Net Loss Per Share Attributable to Common Stockholders - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	6,891,123	6,086,939	21,630,281
Redeemable convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			17,507,788
Convertible preferred stock (on an as-converted basis)
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			192,473
Convertible preferred stock warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share			64,178
Options to purchase common stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	6,891,123	6,086,939	3,865,842

Net Loss Per Share Attributable to Common Stockholders - Reconciliation of Numerator and Denominator Used in Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ 621	$ (10,247)	$ (25,216)	$ (8,257)	$ (14,033)	$ (14,662)	$ (14,176)	$ (16,029)	$ (43,099)	$ (58,900)	$ (35,374)
Accretion to redemption value and cumulative dividends on preferred stock											(6,705)
Net loss attributable to common stockholders									$ (43,099)	$ (58,900)	$ (42,079)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted									37,166,830	36,234,732	8,595,247
Net loss per share attributable to common stockholders, basic and diluted									$ (1.16)	$ (1.63)	$ (4.90)

Supplementary Data - Quarterly Financial Data (Unaudited) - Unaudited Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 27,074	$ 24,144	$ 8,752	$ 11,653	$ 6,272	$ 3,454	$ 3,094	$ 2,223	$ 71,623	$ 15,043	$ 7,712
Net income (loss)	$ 621	$ (10,247)	$ (25,216)	$ (8,257)	$ (14,033)	$ (14,662)	$ (14,176)	$ (16,029)	$ (43,099)	$ (58,900)	$ (35,374)
Net loss per share attributable to common stockholders, basic and diluted	$ 0.02	$ (0.28)	$ (0.69)	$ (0.23)	$ (0.39)	$ (0.40)	$ (0.39)	$ (0.44)	$ (1.16)	$ (1.63)	$ (4.90)

Defined Contribution Plan	12 Months Ended
Defined Contribution Plan	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	16. Defined Contribution Plan The Company sponsors a defined contribution plan under Section 401(k) of the Internal Revenue Code covering substantially all full-time U.S. employees. Employee contributions are voluntary and are determined on an individual basis subject to the maximum allowable under federal tax regulations. During the years ended December 31, 2017, 2016 and 2015, the Company made contributions to the plan of $255,000, $201,000 and $25,000, respectively.

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	Oct. 23, 2019

Defined Contribution Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Company contributions to defined contribution plan	$ 255,000	$ 201,000	$ 25,000

Fair Value Measurements and Investments - Additional Information (Details)	Dec. 31, 2017 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, 2017
Subsequent Events [Abstract]
License termination period	60 days