Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2016	Feb. 28, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2016
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			$ 197.3
Entity Common Stock, Shares Outstanding		36,518,184

BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 104,645	$ 59,822
Short-term investments	77,293	126,889
Accounts receivable	2,159	372
Related party accounts receivable	154	372
Prepaid expenses and other current assets	3,896	2,299
Total current assets	188,147	189,754
Property and equipment, net	4,392	3,481
Intangible assets	1,750	1,750
Goodwill	949	949
Restricted cash	917	917
Other assets	2,973	364
Total assets	199,128	197,215
Current liabilities:
Accounts payable	6,596	4,697
Accrued liabilities	8,824	4,912
Deferred revenues, current portion	20,347	6,130
Total current liabilities	35,767	15,739
Deferred revenue, net of current portion	83,803	54,703
Deferred tax liability	513	507
Other long-term liabilities	566	198
Total liabilities	120,649	71,147
Commitments and contingencies (Note 11)
Stockholders' equity
Common stock, $0.00001 par value; 75,000,000 shares authorized at December 31, 2016 and 2015; 36,490,169 and 36,033,209 shares issued and outstanding at December 31, 2016 and 2015, respectively	1	1
Stockholders notes receivable		(78)
Additional paid-in capital	254,871	243,687
Accumulated other comprehensive loss	(27)	(76)
Accumulated deficit	(176,366)	(117,466)
Total stockholders' equity	78,479	126,068
Total liabilities, convertible preferred stock and stockholders' equity	199,128	197,215
Convertible Preferred Stock
Stockholders' equity
Convertible preferred stock, $0.00001 par value; 10,000,000 shares authorized at December 31, 2016 and 2015; no shares issued and outstanding at December 31, 2016 and 2015, respectively

BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 10, 2015
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	75,000,000	75,000,000
Common stock, shares issued	36,490,169	36,033,209
Common stock, shares outstanding	36,490,169	36,033,209
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Revenues	$ 12,845	$ 5,941	$ 2,751
Revenues from related parties	2,198	1,771	2,326
Total revenues	15,043	7,712	5,077
Operating expenses:
Research and development	54,755	28,357	28,302
General and administrative	19,874	12,558	6,540
Total operating expenses	74,629	40,915	34,842
Loss from operations	(59,586)	(33,203)	(29,765)
Interest income	2,425	1,315	7
Interest expense	(1,689)	(1,732)	(487)
Other income (expense), net	(69)	(1,744)	(55)
Loss before provision for (benefit from) income taxes	(58,919)	(35,364)	(30,300)
Provision for (benefit from) income taxes	(19)	10	10
Net loss	(58,900)	(35,374)	(30,310)
Accretion to redemption value and cumulative dividends on preferred stock		(6,705)	(4,566)
Net loss attributable to common stockholders	$ (58,900)	$ (42,079)	$ (34,876)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.63)	$ (4.90)	$ (35.25)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	36,234,732	8,595,247	989,453
Other comprehensive loss:
Changes in unrealized gain (losses) on investments	$ 49	$ (76)
Total other comprehensive income (loss)	49	(76)
Comprehensive loss	$ (58,851)	$ (35,450)	$ (30,310)

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, 2013							$ 44,244				$ 474
Temporary equity, beginning balance, shares at Dec. 31, 2013							11,995,481				244,782
Beginning balance at Dec. 31, 2013	$ (44,279)	$ 1	$ (399)			$ (43,881)
Beginning balance, shares at Dec. 31, 2013		990,514
Issuance of preferred stock, net of issuance costs								$ 15,808	$ 11,618
Issuance of preferred stock, net of issuance costs, shares								3,107,701	3,355,107
Exercise of stock options	$ 8			$ 8
Exercise of stock options, shares	6,006	6,006
Interest on stockholder notes	$ (5)		(5)
Vesting of early exercise stock option	58			58
Stock-based compensation	553			553
Accretion to redemption value and cumulative dividends on preferred stock							$ 4,566
Accretion to redemption value and cumulative dividends on preferred stock	(4,566)			(619)		(3,947)
Net loss	(30,310)					(30,310)
Temporary equity, ending balance at Dec. 31, 2014							$ 76,236				$ 474
Temporary equity, ending balance, shares at Dec. 31, 2014							18,458,289				244,782
Ending balance at Dec. 31, 2014	(78,541)	$ 1	(404)			(78,138)
Ending balance, shares at Dec. 31, 2014		996,520
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)
Temporary equity, ending balance, shares at Dec. 31, 2015	0
Ending balance at Dec. 31, 2015	$ 126,068	$ 1	(78)	243,687	(76)	(117,466)
Ending balance, shares at Dec. 31, 2015		36,033,209
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
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 at Dec. 31, 2016	$ 78,479	$ 1		$ 254,871	$ (27)	$ (176,366)
Ending balance, shares at Dec. 31, 2016		36,490,169

STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK, CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
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 B-1
Issuance of temporary equity, adjustments to convertible preferred stock liability		$ 1,303
Temporary equity stock issued issuance costs		33
Redeemable Convertible Preferred Stock \| Series C
Issuance of temporary equity, adjustments to convertible preferred stock liability		395
Temporary equity stock issued issuance costs	30	$ 298
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, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (58,900)	$ (35,374)	$ (30,310)
Adjustments to reconcile net loss to net cash (used) provided by operating activities:
(Gain)/loss on disposal of property and equipment	(47)	25
Depreciation and amortization	1,733	1,206	783
Amortization of debt discount		80	40
Accretion of discount on short-term investments	1,662	1,186
Stock-based compensation expense	10,095	3,986	553
Issuance of common stock in connection with services	159
Deferred income taxes	6	8	8
Changes in operating assets and liabilities
Accounts receivable	(1,787)	1,131	(1,638)
Related party accounts receivable	218
Prepaid expenses and other current assets	(1,597)	(1,491)	(261)
Other assets	(2,609)	128	(344)
Accounts payable	1,765	2,944	660
Deferred revenue	43,317	(6,130)	61,463
Accrued liabilities and other liabilities	3,953	3,170	793
Net cash (used in)/provided by operating activities	(2,032)	(27,415)	31,802
Cash flows from investing activities:
Purchases of property and equipment	(2,176)	(1,594)	(1,663)
Proceeds from sales of assets	52
Purchases of investments	(121,517)	(250,901)
Maturities of investments	169,500	122,750
Increase in restricted cash		(817)
Net cash provided by/(used in) investing activities	45,859	(130,562)	(1,663)
Cash flows from financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		74,430	26,802
Proceeds from issuance of common stock under equity incentive plans	930	263	8
Proceeds from initial public offering, net of issuance costs		81,777
Repayment of notes payable	78	(3,067)	(1,256)
Payment of deferred offering costs	(12)
Net cash provided by financing activities	996	153,403	25,554
Net increase/(decrease) in cash and cash equivalents	44,823	(4,574)	55,693
Cash and cash equivalents, beginning of year	59,822	64,396	8,703
Cash and cash equivalents, end of year	104,645	59,822	64,396
Supplemental disclosures of noncash investing and financing items:
Purchases of property and equipment in accounts payable and accrued liabilities	$ 473	100	68
Accretion to redemption value and cumulative dividends on preferred stock		6,705	4,566
Convertible preferred stock liability recorded in connection with redeemable convertible preferred stock		1,509	908
Stock issuance costs in accounts payable and accrued liabilities		13	284
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	13
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	$ 42

Description of the Business	12 Months Ended
Description of the Business	Dec. 31, 2016
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 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. Reverse Stock Split On October 2, 2015, the Company effected a one-for-62.997 reverse stock split of the Company’s issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock. The par values of the common stock, redeemable convertible preferred stock and convertible preferred stock were not adjusted as a result of the reverse stock split. All authorized and issued and outstanding shares of common stock, redeemable convertible preferred stock and convertible preferred stock and per share amounts contained in the accompanying financial statements have been retroactively adjusted to reflect this reverse stock split for all periods presented.

Liquidity	12 Months Ended
Liquidity	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Liquidity	2. Liquidity Since inception, the Company has incurred recurring net operating losses. As December 31, 2016 and 2015, the Company had an accumulated deficit of $176.4 million and $117.5 million, respectively, and expects to incur losses for the foreseeable future. To date, the Company has financed its operations primarily through sales of its common stock in conjunction with the IPO, sales of its convertible preferred securities prior to the IPO and payments received under its collaboration agreements. As of December 31, 2016, the Company had cash, cash equivalents and short-term investments of $181.9 million. In May and June 2015, the Company received aggregate net proceeds of $73.2 million from the issuance of its Series C and Series D redeemable convertible preferred stock. In October 2015, the Company consummated its IPO and raised net proceeds of approximately $81.8 million, after deducting underwriting discounts and commissions and offering expenses.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

3. 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 who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows:

	Revenue									Accounts Receivable, net
	For the Year Ended December 31,									December 31,
	2016			2015			2014			2016			2015
Customer A		64	%		77	%		54	%		93	%		50	%
Customer B		22	%		0	%		0	%		0	%		0	%
Customer C		14	%		23	%		46	%		7	%		50	%

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

Deferred Offering Costs

Deferred offering costs, which consisted primarily of direct incremental costs related to the Company’s initial public offering of its common stock, were capitalized in other assets until the consummation of the initial public offering. These offering costs were reclassified to additional paid-in capital upon the closing of the initial public offering in October 2015.

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, who is the Company’s chief operating decision maker. 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 (“IPR&D”). The Company assesses impairment indicators annually 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, 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 periods presented in these financial statements.

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 include 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.

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 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 to the Company’s technology, (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.

Research and Development Expenses

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.

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 straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, 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.

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 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. A valuation allowance is provided against deferred tax assets unless it is more likely than not that they will be realized.

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. 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.

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, which is the effective date for public companies. Early application is permitted as of January 1, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. Additionally, in March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarifies the implementation guidance on principal versus agent considerations in ASU No. 2014-09. In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies certain aspects of identifying performance obligations and licensing implementation guidance. In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which relates to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. These standards have the same effective date and transition date of January 1, 2018. Presently, the Company plans to adopt this standard in the first quarter of 2018 but have not determined which transition method it will choose. The Company will continue to review the impact that the new standard will have on its financial statement 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 March 2016, the FASB issued ASU No. 2016-09, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting. ASU 2016-09 simplifies several aspects of accounting for share-based payment award transactions, including income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows. ASU 2016-09 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2016, with early adoption permitted. The Company plans to adopt this guidance beginning with its first quarter ending March 31, 2017.

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 us on January 1, 2020. The adoption of this standard is not expected to have a material impact on our financial position or results of operations.

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 addresses the classification of certain specific cash flow issues including debt prepayment or extinguishment costs, 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 is currently evaluating the effect this ASU will have on its statement of cash flows.

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 is currently evaluating the effect this ASU will have on its statement of cash flows.

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.

Fair Value Measurements and Short-term Investments

12 Months Ended

Dec. 31, 2016

Fair Value Disclosures [Abstract]

Fair Value Measurements and Short-term Investments

4. Fair Value Measurements and Short-Term Investments

The Company records its financial assets and liabilities at fair value. The accounting guidance for fair value provides a framework for measuring fair value, clarifies the definition of fair value, and expands disclosures regarding fair value measurements. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The accounting guidance establishes a three-tiered hierarchy, which prioritizes the inputs used in the valuation methodologies in measuring 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 financial assets and liabilities subject to fair value measurements on a recurring basis and the level of inputs used in such measurements (in thousands):

	December 31, 2016
	Level I		Level II		Level III		Total
Assets
Money market funds	$	89,626	$	—	$	—	$	89,626
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		77,293		—		77,293
	$	90,543	$	77,293	$	—	$	167,836

	December 31, 2015
	Level I		Level II		Level III		Total
Assets
Money market funds	$	44,714	$	—	$	—	$	44,714
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		140,392		—		140,392
	$	45,631	$	140,392	$	—	$	186,023

The following tables set forth the gross unrealized gains and losses on the Company’s investments (in thousands):

	December 31, 2016
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
U.S. Government bonds	$	77,295	$	8	$	(10	)	$	77,293
Total securities	$	77,295	$	8	$	(10	)	$	77,293

	December 31, 2015
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
U.S. Government bonds	$	126,965	$	—	$	(76	)	$	126,889
Total securities	$	126,965	$	—	$	(76	)	$	126,889

The following tables set forth the contractual maturities of securities classified as available-for-sale (in thousands):

	December 31, 2016
Due within one year	$	77,293
Total	$	77,293

	December 31, 2015
Due within one year	$	126,889
Total	$	126,889

- 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, 2016

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

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

	December 31
	2016			2015
Machinery and equipment	$	5,973		$	4,910
Computer equipment and software		888			452
Furniture and fixtures		651			51
Leasehold improvements		578			720
Construction in progress		45			169
		8,135			6,302
Less: accumulated depreciation and amortization		(3,743	)		(2,821	)
	$	4,392		$	3,481

Depreciation and amortization expense was $1.7 million, $1.2 million and $783,000 for the years ended December 31, 2016, 2015 and 2014, respectively.

Goodwill and Intangible Assets

12 Months Ended

Dec. 31, 2016

Goodwill And Intangible Assets Disclosure [Abstract]

Goodwill and Intangible Assets

6. Goodwill and Intangible Assets

Goodwill and in-process research and development assets result 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 technology platform and is accounted for as an indefinite-lived intangible asset until the underlying project is completed or abandoned.

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

	December 31,
	2016		2015
Goodwill	$	949	$	949
In-process research and development		1,750		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, 2016

Accrued Liabilities Current [Abstract]

Accrued Liabilities

7. Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2016		2015
Payroll and related expenses	$	3,971	$	2,839
Research and clinical expenses		3,909		1,562
Property and equipment		331		—
Legal and professional expenses		264		296
Other accrued expenses		349		215
Total	$	8,824	$	4,912

Research and Collaboration Agreements	12 Months Ended
Research and Collaboration Agreements	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Research and Collaboration Agreements	8. 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. Under the terms of the Discovery Agreement, AbbVie receives exclusive worldwide rights to develop and commercialize PDC against up to two undetermined targets. The Company shall perform research services to discover the Probodies 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, (2) the research services related to CD71 Probody, (3) the obligation to participate in the CD71 Agreement joint research committee, (4) the research services related to the first discovery target (5) the research, development and commercialization license for the first discovery target, and (6) the obligation to participate in the Discovery Agreement joint research committee. The Company concluded that, at the inception of the agreement, AbbVie’s option for the second discovery target is substantive and does not represent a deliverable of the agreement. The Company determined that the research, development and commercialization license for CD71 and discovery targets do not have a standalone value without the Company’s respective research services and expertise. The Company considered factors such as novelty of the Probody 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 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 $3.2 million for the year ended December 31, 2016, related to the AbbVie Agreements. As of December 31, 2016, deferred revenue related to the CD71 unit of accounting was $17.7 million and $8.9 million related to the Discovery Agreement unit of accounting. 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 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 will have additional rights to substitute up to two collaboration targets. 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 was initially entitled to receive contingent payments of up to an aggregate of $1,217.0 million as follows: (i) up to $25.0 million for additional targets; (ii) up to $114.0 million in development milestone payments per research target program or up to $456.0 million if the maximum of four research targets are selected; (iii) up to $124.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program or up to $496.0 million if the maximum of four research targets are selected, and (iv) up to $60.0 million in sales milestones payments per research target program or up to $240.0 million if maximum of four research targets are selected. The Company is entitled to royalty payments in the mid to high single digits to low teens 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. Accordingly, any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment will be recognized as revenue in full upon triggering the event. 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. This milestone payment was recognized as revenue in its entirety upon the selection because the achievement of this milestone was based on the Company’s performance. The Company recognized revenue of $9.6 million, $5.9 million and $2.8 million for the years ended December 31, 2016, 2015 and 2014, respectively. As of December 31, 2016 and 2015, deferred revenue relating to the BMS Agreement was $60.9 million and $42.6 million, respectively. The amount due from BMS under the BMS Agreement was $2.2 million and $0.4 million as of December 31, 2016 and 2015, 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 technology to create 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 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. Under the research licenses, the parties have one replacement right for each target, which needs to be made before the third anniversary of the agreement execution. 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. The research activities for a particular target will last until January 2018 unless they are terminated by one of the parties or when a development and commercialization license is obtained with respect to that target. 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. 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 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 will recognize $13.0 million upon delivery of development and commercialization licenses and will recognize amounts allocated to the future technology improvements over the term of the license. The estimated fair value of assets and services received was also $13.2 million, of which $12.7 million was allocated to the licenses received and was charged to research and development expense, with the remaining amount of $0.5 million 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. As of December 31, 2016 and 2015, deferred revenue relating to the ImmunoGen Agreement was $13.2 million for both periods. MD Anderson In November 2015, the Company entered into a research collaboration agreement with MD Anderson to research Probody-enabled chimeric antigen receptor killer (CAR-NK) cell therapies, known as ProCAR-NK cell therapies. Under this collaboration, MD Anderson will use the Company’s Probody technology to conduct research of ProCAR-NK cell therapies against certain targets selected by the Company in cancer immunotherapy. Under the research collaboration agreement, the Company has the right to exercise an option, during the option period expiring on November 2, 2019 and upon payment of an option exercise fee, to negotiate and acquire a worldwide, exclusive, sublicensable license from MD Anderson for development and commercialization of products directed against any of the selected targets. The research collaboration agreement 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. The impact of this agreement was not material for the financial statements for the years ended December 31, 2016 and 2015. 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 without a selection. The Pfizer Agreement provides Pfizer with an option to acquire an exclusive development and commercialization license for each research project target. Upon exercise of the option, Pfizer (1) will receive an exclusive development and commercialization license for use of the Probody therapeutic during the development, manufacturing and commercialization of the potential product, and (2) will be responsible for the development, manufacturing and commercialization of such potential products. Pursuant to the Pfizer Agreement, the Company received an upfront payment of $6.0 million and is entitled to receive contingent payments of up to an aggregate of $263.5 million as follows: (i) up to $4.5 million upon exercise of the license options, (ii) up to $38.0 million from the achievement of development milestones for the research target programs, (iii) up to $101.0 million in milestone payments for the first commercial sale in various territories for up to three indications per research target program, and (iv) up to $120.0 million in sales milestones payments for the research target programs. The Company is entitled to receive royalties in the mid-single digit royalties from potential future sales of product candidates. The Company will also receive research and development service fees based on a prescribed FTE rate per year that is capped. In accordance with ASC 605-25, the Company identified the following deliverables at the inception of the Pfizer Agreement: (1) the research license, (2) the research services and (3) the obligation to participate in the joint research committee. The Company determined that the research license does not have stand-alone value to Pfizer due to the specialized nature of the research services to be provided by the Company, and accordingly, this deliverable was combined with the research services and participation in the joint research committee as a single unit of accounting. The Company concluded that, at the inception of the agreement, Pfizer’s options to obtain an exclusive development and commercialization license for each research project target do not represent deliverables of the agreement because they are substantive options and do not contain a significant or incremental discount. The upfront payment of $6.0 million was recorded as deferred revenue and is being recognized on a ratable basis over the estimated performance period of seven years. In December 2014, Pfizer selected an additional target and paid $1.5 million, which was recorded as deferred revenue and is being recognized over the remaining performance period. Following the lapse of the Pfizer’s option to select a fourth target in May 2016, the amortization period of deferred revenue was adjusted to five and a half years. The Company recognized revenue of $2.2 million, $1.8 million and $2.3 million for the year ended December 31, 2016, 2015 and 2014, respectively. As of December 31, 2016 and 2015, deferred revenue relating to the Pfizer Agreement was $3.4 million and $4.9 million, respectively. The amount due from Pfizer under the Agreement was $0.1 million and $0.4 million as of December 31, 2016 and 2015, respectively.

License Agreement	12 Months Ended
License Agreement	Dec. 31, 2016
Research And Development [Abstract]
License Agreement	9. License Agreement The Company has an exclusive, worldwide license agreement (the “UC Agreement”) with the Regents of the University of California (the “UC Regents”), acting through its Santa Barbara Campus, relating to the use of certain patents and technology relating to its core technology, including its therapeutic antibodies. Pursuant to the UC Agreement, the Company is obligated to (i) make royalty payments to the UC Regents on net sales of its products covered under the agreement, subject to annual minimum amounts, (ii) make milestone payments to the UC Regents upon the occurrence of certain events, (iii) make a milestone payment to the UC Regents upon occurrence of an IPO or change of control, and (iv) reimburse the UC Regents for prosecution and maintenance of the licensed patents. If the Company sublicenses its rights under the UC Agreement, it is obligated to pay the UC Regents a percentage of the total gross proceeds received in consideration of the grant of the sublicense, which total amount would be first reduced by the aggregate amount of certain research and development related expenses incurred by the Company. In 2013, the Company amended the UC Agreement to reduce the amounts due to the UC Regents upon receipt by the Company of upfront payments, milestone payments and royalties from sublicensees. In exchange for this amendment, the Company issued to the UC Regents 157,332 shares of common stock. The UC 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, 2016, 2015 and 2014, the Company incurred expenses of $2.1 million, $0.3 million and $0.7 million respectively, to the UC Regents under the provisions of the UC Agreement. Royalty obligations The Company has annual minimum royalty obligations of $150,000 under the terms of certain exclusive licensed patent rights.

Long-term Debt	12 Months Ended
Long-term Debt	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-term Debt	10. 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 bear interest at 11.74% per annum. 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 our 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 is amortized to interest expense using the effective interest method over the term of the Debt Facility. The warrant liability is subject to remeasurement to fair value at each balance sheet date until the earliest of the exercise or expiration of the convertible preferred stock warrant, 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

11. Commitments and Contingencies

Operating Lease

New Lease Agreement

On December 10, 2015, the Company entered into a lease (the “New 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 New Lease commenced on October 1, 2016. The New 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 New Lease.

The New 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 is entitled to a one-time improvement allowance of up to $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 New 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.

Amendment to Existing/Current Lease

In March 2016, the Company entered into an agreement to terminate its existing/current lease, which was due to expire on January 31, 2019 (“Lease Termination”) with its current landlord. The Lease Termination provided for an early termination of the existing/current lease effective on November 30, 2016. The Company will not be required to pay the landlord a termination payment in connection with the early termination of the existing/current lease.

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:
2017	$	3,387
2018		4,374
2019		4,506
2020		4,641
2021 and beyond		29,503
Total	$	46,411

Rent expense during the years ended December 31, 2016, 2015 and 2014 was $1.8 million, $0.9 million and $0.8 million, respectively.

Legal Proceedings

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

Indemnifications

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

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

Convertible Preferred Stock	12 Months Ended
Convertible Preferred Stock	Dec. 31, 2016
Equity [Abstract]
Convertible Preferred Stock	12. 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, 2016 and 2015.

Common Stock

12 Months Ended

Dec. 31, 2016

Equity [Abstract]

Common Stock

13. 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, 2016 and 2015, 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,
	2016		2015
Options issued and outstanding		6,158,746		5,270,751
Shares available for future stock option grants		2,493,188		2,401,406
		8,651,934		7,672,157

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(e))
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Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(31))
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
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Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

+ Details

Stock Option Plans

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Option Plans

14. Stock Option Plans

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, 2016, 2,493,188 shares of common stock were available for future issuance under the 2015 Plan.

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, 2013		776,628			1,686,490		$	1.071
Options authorized		1,587,377			—			—
Options granted		(697,557	)		697,557			1.449
Options exercised		—			(6,006	)		1.260
Options forfeited		230,169			(230,169	)		0.945
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			3.694
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			5.932		8.0	$	34,621
Options Exercisable—December 31, 2016					3,071,643			3.750		7.3	$	23,042
Options vested and expected to vest—December 31, 2016					6,078,576			5.897		8.0	$	34,351

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, 2016, 2015 and 2014, respectively.

The aggregate intrinsic value of stock options exercised in the years ended December 31, 2016, 2015 and 2014 was $4.6 million, $2.3 million and $1,500, respectively.

The following table summarizes information about stock options outstanding and exercisable by exercise price at December 31, 2016:

		Outstanding				Exercisable
Range of Exercise Price		Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable
$0.945 - $1.1339			742,675		5.26		738,002
$1.26 - $1.4489			622,789		6.61		568,230
$1.512 - $1.5119			233,999		7.40		144,604
$1.575 - $1.5749			778,456		8.11		499,013
$4.473 - $4.4728			458,799		8.35		205,890
$6.615 - $6.6147			1,940,153		8.62		640,426
$10.17 - $12.28			463,195		9.51		61,704
$	14.06		2,000		9.16		—
$	14.46		900,246		9.02		209,089
$	21.51		16,434		8.94		4,685
			6,158,746				3,071,643

The following table summarizes information about stock options outstanding and vested by exercise price at December 31, 2015:

	Outstanding				Exercisable
Exercise Price	Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable
$0.945		411,576		7.16		278,861
$1.134		572,079		5.61		572,079
$1.260		342,793		7.05		306,083
$1.386		164,844		8.05		148,970
$1.449		249,886		8.32		183,100
$1.512		240,058		8.78		81,281
$1.575		815,030		9.13		160,073
$4.473		464,799		9.35		24,629
$6.615		1,966,357		9.63		126,667
$12.00		26,895		9.77		26,895
$21.51		16,434		9.95		—
		5,270,751				1,908,638

The options granted in the years ended December 31, 2016, 2015 and 2014 had a weighted average per share grant-date fair value of $8.936, $7.169, and $0.945, respectively. At December 31, 2016, the unrecognized compensation expense with respect to options granted to employees was $23.4 million, and is expected to be recognized over 2.5 years, respectively.

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	12 Months Ended
Employee Stock Purchase Plan	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Employee Stock Purchase Plan	15. 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 31,564 shares of common stock under the ESPP in 2016. Shares available for future purchase under the ESPP were 683,234 at December 31, 2016. The compensation expense related to the ESPP was $145,000 and $0 for the years ended December 31, 2016 and 2015, respectively. As of December 31, 2016, there was $113,000 of unrecognized compensation cost related to the ESPP, which we expect to recognize over 5 months.

Stock Based Compensation

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock Based Compensation

16. Stock Based Compensation

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

	Year Ended December 31,
	2016		2015		2014
Research and development	$	4,925	$	1,972	$	195
General and administrative		5,170		2,014		358
Total stock-based compensation expense	$	10,095	$	3,986	$	553

Stock-based compensation expense for employees was $9.4 million, $3.2 million and $0.5 million for the years ended December 31, 2016, 2015 and 2014, 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 $0.9 million, $0.8 million and $0.1 million for the years ended December 31, 2016, 2015 and 2014, 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,
	2016	2015	2014	2016
Expected volatility	76.4% – 83.5%	62.9% – 68.9%	66.4% – 71.2%	50.4% – 75.6%
Risk-free interest rate	1.2% – 2.1%	1.4% – 1.9%	1.6% – 2.0%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %
Expected term (in years)	5.3 – 5.9	5.2 – 7.2	5.3 – 6.1	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, as the Company did not have sufficient historical performance to develop reasonable expectations about future exercise patterns and post-vesting employment information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior. The expected term of the ESPP right is equal to the six-month look-back period.

Expected Volatility. The expected stock price volatility for the Company’s stock options was determined by examining the historical volatilities for comparable publicly traded companies within the biotechnology and pharmaceutical industry using an average of historical volatilities of Company’s industry peers. Volatility for ESPP rights 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.

Expected Forfeiture Rate. The forfeiture rates were estimated based on actual employee head count and were immaterial to the financial statements during 2016, 2015 and 2014.

- Definition

The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details.

+ References + Details

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2016
Related Party Transactions [Abstract]
Related Party Transactions Disclosure	17. Related Party Transactions Certain employees of Third Rock Ventures, a stockholder of the Company, provide consulting services to the Company. General and administrative expenses for these services were $48,000, $33,000, $46,000 for the year ended December 31, 2016, 2015 and 2014, respectively. The amounts outstanding and included in accounts payable were $12,000 and $0 as of December 31, 2016 and 2015, respectively. Prior to the Company’s IPO, it entered into full recourse loans (“stockholder notes” or “loans”) with current and former executive officers. Principal and interest under these loans are due at the earliest of (i) the fifth anniversary of the related note, (ii) the sale of the shares securing the notes, or (iii) thirty days after the termination of services. The principal loan amount and the accrued interest are reported as a deduction from stockholders’ equity (deficit) on the Company’s balance sheets. These loans were repaid and terminated in August 2015 and April 2016. There was no outstanding balance at December 31, 2016 and approximately $78,000 was outstanding at December 31, 2015, respectively. Interest income earned on the loans was immaterial during the years ended December 31, 2016, 2015 and 2014. 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, $1.8 million and $2.3 million for the years ended December 31, 2016, 2015 and 2014, respectively (Note 8). As of December 31, 2016 and 2015, deferred revenue relating to the Pfizer Agreement was $3.4 million and $4.9 million, respectively. The amount due from Pfizer under the agreement was $0.1 million and $0.4 million as of December 31, 2016 and 2015, respectively.

Income Taxes

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Income Taxes

18. 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,
	2016			2015		2014
Current:
Federal	$	—		$	—	$	—
State		1			2		1
Total current		1			2		1
Deferred:
Federal		(20	)		8		9
State					—		—
Total deferred		(20	)		8		9
Provision for (benefit from) income taxes	$	(19	)	$	10	$	10

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

	Years Ended December 31,
	2016			2015			2014
U.S. federal taxes at statutory rate		34.0	%		34.0	%		34.0	%
State tax, net of federal benefit		0.8	%		0.8	%		1.0	%
Stock compensation		(0.6	)%		(1.1	)%		(0.3	)%
Tax attributes subject to 382 limitation		0.0	%		(35.4	)%		0.0	%
Tax credits		2.2	%		0.8	%		1.3	%
Other		(0.8	)%		(1.8	)%		(0.1	)%
Change in valuation allowance		(35.6	)%		2.7	%		(35.9	)%
Total		—	%		—	%		—	%

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,
	2016			2015			2014
Net operating loss carryforwards	$	24,528		$	5,688		$	22,484
Research and development credits		2,683			1,337			2,009
Intangible—in-process R&D		81			88			96
Deferred revenue		13,857			16,182			1,579
Accruals and deferred rent		1,335			998			276
Stock-based compensation		3,963			1,125			155
Other		1			26			—
Total gross deferred income tax assets		46,448			25,444			26,599
Less: valuation allowance		(46,137	)		(25,043	)		(26,012	)
Deferred tax assets, net of valuation allowance		311			401			587
Fixed assets		(229	)		(313	)		(491	)
In-process R&D		(595	)		(595	)		(595	)
Deferred tax liabilities		(824	)		(908	)		(1,086	)
Net deferred income tax liabilities	$	(513	)	$	(507	)	$	(499	)

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, 2016 and 2015 was an increase of $21.1 million and a decrease of $1.0 million, respectively and for the year ended December 31, 2014 was an increase of $10.9 million.

The Company has net operating loss carryforwards for federal and state income tax purposes of approximately $71.5 million and $14.3 million, respectively, as of December 31, 2016 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 $1.7 million and $2.9 million, respectively, as of December 31, 2016 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.

Internal Revenue Code 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 2015. As a result, the federal and state carryforwards associated with the NOL and credit deferred tax assets were reduced by the amount of tax attributes estimated to expire during their respective carryforward periods. There may be further ownership changes after December 31, 2016.

The Company had approximately $1.2 million of unrecognized tax benefits as of December 31, 2016, 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,
	2016		2015			2014
Balance at the beginning of the year	$	666	$	3,019		$	532
Additions based on tax positions related to current year		23		(2,312	)		2,473
Adjustment based on submitted prior year tax returns		493		(41	)		14
Balance at end of the year	$	1,182	$	666		$	3,019

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, 2016, 2015 and 2014.

The Company files income tax returns in the United States, including California state jurisdiction. The tax years 2010 to 2016 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, 2016, the Company is not under examination by the Internal Revenue Service or any state or foreign tax jurisdiction.

Defined Contribution Plan	12 Months Ended
Defined Contribution Plan	Dec. 31, 2016
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	19. 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, 2016, 2015 and 2014, the Company made contributions to the plan of $201,000, $25,000 and $16,500, respectively.

Net Loss Per Share Attributable to Common Stockholders

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

Net Loss Per Share Attributable to Common Stockholders

20. 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,
	2016		2015		2014
Redeemable convertible preferred stock (on an as-converted basis)		—		17,507,788		15,024,251
Convertible preferred stock (on an as-converted basis)		—		192,473		244,782
Options to purchase common stock		6,086,939		3,865,842		1,987,532
Convertible preferred stock warrants		—		64,178		81,620
Total		6,086,939		21,630,281		17,338,185

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,
	2016			2015			2014
Numerator:
Net loss	$	(58,900	)	$	(35,374	)	$	(30,310	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			(6,705	)		(4,566	)
Net loss attributable to common stockholders		(58,900	)		(42,079	)		(34,876	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		36,234,732			8,595,247			989,453
Net loss per share attributable to common stockholders, basic and diluted	$	(1.63	)	$	(4.90	)	$	(35.25	)

Supplementary Data - Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Supplementary Data – Quarterly Financial Data (Unaudited)

21. Supplementary Data – Quarterly Financial Data (Unaudited)

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2016 and 2015:

	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 attributable to common stockholders	$	(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	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2015			September 30, 2015			June 30, 2015			March 31, 2015
Revenue	$	1,988		$	1,939		$	2,043		$	1,742
Net loss	$	(11,487	)	$	(11,870	)	$	(5,806	)	$	(6,211	)
Net loss attributable to common stockholders	$	(12,045	)	$	(14,828	)	$	(7,563	)	$	(7,643	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.39	)	$	(14.26	)	$	(7.56	)	$	(7.67	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2016

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

Customers who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows:

	Revenue									Accounts Receivable, net
	For the Year Ended December 31,									December 31,
	2016			2015			2014			2016			2015
Customer A		64	%		77	%		54	%		93	%		50	%
Customer B		22	%		0	%		0	%		0	%		0	%
Customer C		14	%		23	%		46	%		7	%		50	%

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

Deferred Offering Costs

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

Research and Development Expenses

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

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Schedule of Customers who Represent 10% or More of Total Revenue or Net Accounts Receivable Balance

Customers who represent 10% or more of the Company’s total revenue during each period presented or net accounts receivable balance at each respective balance sheet date are as follows:

	Revenue									Accounts Receivable, net
	For the Year Ended December 31,									December 31,
	2016			2015			2014			2016			2015
Customer A		64	%		77	%		54	%		93	%		50	%
Customer B		22	%		0	%		0	%		0	%		0	%
Customer C		14	%		23	%		46	%		7	%		50	%

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, 2016

Fair Value Disclosures [Abstract]

Schedule of Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis

	December 31, 2016
	Level I		Level II		Level III		Total
Assets
Money market funds	$	89,626	$	—	$	—	$	89,626
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		77,293		—		77,293
	$	90,543	$	77,293	$	—	$	167,836

	December 31, 2015
	Level I		Level II		Level III		Total
Assets
Money market funds	$	44,714	$	—	$	—	$	44,714
Restricted cash (money market funds)		917		—		—		917
U.S. Government bonds		—		140,392		—		140,392
	$	45,631	$	140,392	$	—	$	186,023

Schedule of Gross Unrealized Gains and Losses on the Company's Investments

The following tables set forth the gross unrealized gains and losses on the Company’s investments (in thousands):

	December 31, 2016
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
U.S. Government bonds	$	77,295	$	8	$	(10	)	$	77,293
Total securities	$	77,295	$	8	$	(10	)	$	77,293

	December 31, 2015
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
U.S. Government bonds	$	126,965	$	—	$	(76	)	$	126,889
Total securities	$	126,965	$	—	$	(76	)	$	126,889

Schedule of Contractual Maturities of Securities Classified as Available-for-sale

The following tables set forth the contractual maturities of securities classified as available-for-sale (in thousands):

	December 31, 2016
Due within one year	$	77,293
Total	$	77,293

	December 31, 2015
Due within one year	$	126,889
Total	$	126,889

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2016

Property Plant And Equipment [Abstract]

Summary of Property and Equipment, Net

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

	December 31
	2016			2015
Machinery and equipment	$	5,973		$	4,910
Computer equipment and software		888			452
Furniture and fixtures		651			51
Leasehold improvements		578			720
Construction in progress		45			169
		8,135			6,302
Less: accumulated depreciation and amortization		(3,743	)		(2,821	)
	$	4,392		$	3,481

Goodwill and Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2016

Goodwill And Intangible Assets Disclosure [Abstract]

Schedule of Goodwill and Intangible Assets

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

	December 31,
	2016		2015
Goodwill	$	949	$	949
In-process research and development		1,750		1,750

Accrued Liabilities (Tables)

12 Months Ended

Dec. 31, 2016

Accrued Liabilities Current [Abstract]

Schedule of Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2016		2015
Payroll and related expenses	$	3,971	$	2,839
Research and clinical expenses		3,909		1,562
Property and equipment		331		—
Legal and professional expenses		264		296
Other accrued expenses		349		215
Total	$	8,824	$	4,912

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:
2017	$	3,387
2018		4,374
2019		4,506
2020		4,641
2021 and beyond		29,503
Total	$	46,411

Common Stock (Tables)

12 Months Ended

Dec. 31, 2016

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,
	2016		2015
Options issued and outstanding		6,158,746		5,270,751
Shares available for future stock option grants		2,493,188		2,401,406
		8,651,934		7,672,157

- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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+ Details

Stock Option Plans (Tables)

12 Months Ended

Dec. 31, 2016

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, 2013		776,628			1,686,490		$	1.071
Options authorized		1,587,377			—			—
Options granted		(697,557	)		697,557			1.449
Options exercised		—			(6,006	)		1.260
Options forfeited		230,169			(230,169	)		0.945
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			3.694
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			5.932		8.0	$	34,621
Options Exercisable—December 31, 2016					3,071,643			3.750		7.3	$	23,042
Options vested and expected to vest—December 31, 2016					6,078,576			5.897		8.0	$	34,351

Summary of Stock Options Outstanding and Exercisable by Exercise Price

The following table summarizes information about stock options outstanding and exercisable by exercise price at December 31, 2016:

		Outstanding				Exercisable
Range of Exercise Price		Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable
$0.945 - $1.1339			742,675		5.26		738,002
$1.26 - $1.4489			622,789		6.61		568,230
$1.512 - $1.5119			233,999		7.40		144,604
$1.575 - $1.5749			778,456		8.11		499,013
$4.473 - $4.4728			458,799		8.35		205,890
$6.615 - $6.6147			1,940,153		8.62		640,426
$10.17 - $12.28			463,195		9.51		61,704
$	14.06		2,000		9.16		—
$	14.46		900,246		9.02		209,089
$	21.51		16,434		8.94		4,685
			6,158,746				3,071,643

The following table summarizes information about stock options outstanding and vested by exercise price at December 31, 2015:

	Outstanding				Exercisable
Exercise Price	Number Outstanding		Weighted-Average Remaining Contractual Life (Years)		Number Exercisable
$0.945		411,576		7.16		278,861
$1.134		572,079		5.61		572,079
$1.260		342,793		7.05		306,083
$1.386		164,844		8.05		148,970
$1.449		249,886		8.32		183,100
$1.512		240,058		8.78		81,281
$1.575		815,030		9.13		160,073
$4.473		464,799		9.35		24,629
$6.615		1,966,357		9.63		126,667
$12.00		26,895		9.77		26,895
$21.51		16,434		9.95		—
		5,270,751				1,908,638

Stock Based Compensation (Tables)

12 Months Ended

Dec. 31, 2016

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Total Stock-based Compensation Recorded Related to Options Granted to Employees and Nonemployees and Employee Stock Purchase Plan

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

	Year Ended December 31,
	2016		2015		2014
Research and development	$	4,925	$	1,972	$	195
General and administrative		5,170		2,014		358
Total stock-based compensation expense	$	10,095	$	3,986	$	553

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,
	2016	2015	2014	2016
Expected volatility	76.4% – 83.5%	62.9% – 68.9%	66.4% – 71.2%	50.4% – 75.6%
Risk-free interest rate	1.2% – 2.1%	1.4% – 1.9%	1.6% – 2.0%	0.5% – 0.6%
Dividend yield	— %	— %	— %	— %
Expected term (in years)	5.3 – 5.9	5.2 – 7.2	5.3 – 6.1	0.5

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

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,
	2016			2015		2014
Current:
Federal	$	—		$	—	$	—
State		1			2		1
Total current		1			2		1
Deferred:
Federal		(20	)		8		9
State					—		—
Total deferred		(20	)		8		9
Provision for (benefit from) income taxes	$	(19	)	$	10	$	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,
	2016			2015			2014
U.S. federal taxes at statutory rate		34.0	%		34.0	%		34.0	%
State tax, net of federal benefit		0.8	%		0.8	%		1.0	%
Stock compensation		(0.6	)%		(1.1	)%		(0.3	)%
Tax attributes subject to 382 limitation		0.0	%		(35.4	)%		0.0	%
Tax credits		2.2	%		0.8	%		1.3	%
Other		(0.8	)%		(1.8	)%		(0.1	)%
Change in valuation allowance		(35.6	)%		2.7	%		(35.9	)%
Total		—	%		—	%		—	%

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,
	2016			2015			2014
Net operating loss carryforwards	$	24,528		$	5,688		$	22,484
Research and development credits		2,683			1,337			2,009
Intangible—in-process R&D		81			88			96
Deferred revenue		13,857			16,182			1,579
Accruals and deferred rent		1,335			998			276
Stock-based compensation		3,963			1,125			155
Other		1			26			—
Total gross deferred income tax assets		46,448			25,444			26,599
Less: valuation allowance		(46,137	)		(25,043	)		(26,012	)
Deferred tax assets, net of valuation allowance		311			401			587
Fixed assets		(229	)		(313	)		(491	)
In-process R&D		(595	)		(595	)		(595	)
Deferred tax liabilities		(824	)		(908	)		(1,086	)
Net deferred income tax liabilities	$	(513	)	$	(507	)	$	(499	)

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,
	2016		2015			2014
Balance at the beginning of the year	$	666	$	3,019		$	532
Additions based on tax positions related to current year		23		(2,312	)		2,473
Adjustment based on submitted prior year tax returns		493		(41	)		14
Balance at end of the year	$	1,182	$	666		$	3,019

Net Loss Per Share Attributable to Common Stockholders (Tables)

12 Months Ended

Dec. 31, 2016

Earnings Per Share [Abstract]

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

	Year Ended December 31,
	2016		2015		2014
Redeemable convertible preferred stock (on an as-converted basis)		—		17,507,788		15,024,251
Convertible preferred stock (on an as-converted basis)		—		192,473		244,782
Options to purchase common stock		6,086,939		3,865,842		1,987,532
Convertible preferred stock warrants		—		64,178		81,620
Total		6,086,939		21,630,281		17,338,185

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

	Year Ended December 31,
	2016			2015			2014
Numerator:
Net loss	$	(58,900	)	$	(35,374	)	$	(30,310	)
Add: accretion to redemption value and cumulative dividends on preferred stock		—			(6,705	)		(4,566	)
Net loss attributable to common stockholders		(58,900	)		(42,079	)		(34,876	)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted		36,234,732			8,595,247			989,453
Net loss per share attributable to common stockholders, basic and diluted	$	(1.63	)	$	(4.90	)	$	(35.25	)

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

12 Months Ended

Dec. 31, 2016

Quarterly Financial Information Disclosure [Abstract]

Unaudited Financial Information

The following table represents certain unaudited financial information for each of the quarters ended December 31, 2016 and 2015:

	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 attributable to common stockholders	$	(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	)

	Three Months Ended
(in thousands, except per share data)	December 31, 2015			September 30, 2015			June 30, 2015			March 31, 2015
Revenue	$	1,988		$	1,939		$	2,043		$	1,742
Net loss	$	(11,487	)	$	(11,870	)	$	(5,806	)	$	(6,211	)
Net loss attributable to common stockholders	$	(12,045	)	$	(14,828	)	$	(7,563	)	$	(7,643	)
Net loss per share attributable to common stockholders, basic and diluted	$	(0.39	)	$	(14.26	)	$	(7.56	)	$	(7.67	)

Description of the Business - Additional Information (Detail) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Oct. 14, 2015 USD ($) $ / shares shares	Oct. 02, 2015	Oct. 31, 2015 USD ($)	Dec. 31, 2016 shares	Dec. 31, 2015 USD ($) shares
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses \| $			$ 81,800		$ 81,777
Reverse stock split				one-for-62.997
Reverse stock split conversion ratio		0.01587
Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
Issuance of common stock, shares				11,000	7,666,667
Initial Public Offering \| Common Stock
Business Acquisition Equity Interests Issued Or Issuable [Line Items]
IPO price per share \| $ / shares	$ 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

Liquidity - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	2 Months Ended	12 Months Ended
	Oct. 31, 2015	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Accumulated deficit			$ (117,466)		$ (176,366)
Cash, cash equivalents, and short-term investments					$ 181,900
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			74,430	$ 26,802
Net proceeds from IPO after underwriting discounts, commissions and estimated offering expenses	$ 81,800		$ 81,777
Series C and Series D Redeemable Convertible Preferred Stock
Organization Consolidation And Presentation Of Financial Statements [Line Items]
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs		$ 73,200

- Definition

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

+ References + Details

Summary of Significant Accounting Policies - Schedule of Customers who Represent 10% or More of Total Revenue During Each Period Presented or Net Accounts Receivable Balance (Details) - Customer Concentration Risk	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	64.00%	77.00%	54.00%
Revenue \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	22.00%	0.00%	0.00%
Revenue \| Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	14.00%	23.00%	46.00%
Accounts Receivable, net \| Customer A
Concentration Risk [Line Items]
Concentration risk, percentage	93.00%	50.00%
Accounts Receivable, net \| Customer B
Concentration Risk [Line Items]
Concentration risk, percentage	0.00%	0.00%
Accounts Receivable, net \| Customer C
Concentration Risk [Line Items]
Concentration risk, percentage	7.00%	50.00%

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) Activity Segment	Dec. 31, 2015 USD ($)	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
Impairment of intangible assets	0	0
Impairment of long-lived assets	0	$ 0
Convertible preferred stock, shares issued \| shares			27,135,453
Interest or penalties charged in relation to unrecognized tax benefits	$ 0
Initial Public Offering \| Common Stock
Summary Of Significant Accounting Policies [Line Items]
Convertible preferred stock, shares issued \| shares			27,135,453

Summary of Significant Accounting Policies - Schedule of Useful Lives of Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2016
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 Financial Assets and Liabilities Subject to Fair Value Measurements on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Assets, fair value	$ 167,836	$ 186,023
Money market funds
Assets
Assets, fair value	89,626	44,714
Restricted cash (money market funds)
Assets
Assets, fair value	917	917
U. S. Government bonds
Assets
Assets, fair value	77,293	140,392
Level I
Assets
Assets, fair value	90,543	45,631
Level I \| Money market funds
Assets
Assets, fair value	89,626	44,714
Level I \| Restricted cash (money market funds)
Assets
Assets, fair value	917	917
Level II
Assets
Assets, fair value	77,293	140,392
Level II \| U. S. Government bonds
Assets
Assets, fair value	$ 77,293	$ 140,392

Fair Value Measurements and Investments - Schedule of Gross Unrealized Gains and Losses on the Company's Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	$ 77,295	$ 126,965
Gross Unrealized Holding Gains	8
Gross Unrealized Holding Losses	(10)	(76)
Aggregate Fair Value	77,293	126,889
U. S. Government bonds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Amortized Cost	77,295	126,965
Gross Unrealized Holding Gains	8
Gross Unrealized Holding Losses	(10)	(76)
Aggregate Fair Value	$ 77,293	$ 126,889

Fair Value Measurements and Investments - Schedule of Contractual Maturities of Securities Classified as Available-for-sale (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Due within one year	$ 77,293	$ 126,889
Total	$ 77,293	$ 126,889

Property and Equipment - Summary of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property Plant And Equipment [Line Items]
Total property and equipment	$ 8,135	$ 6,302
Less: accumulated depreciation and amortization	(3,743)	(2,821)
Property and equipment, net	4,392	3,481
Machinery and equipment
Property Plant And Equipment [Line Items]
Total property and equipment	5,973	4,910
Computer equipment and software
Property Plant And Equipment [Line Items]
Total property and equipment	888	452
Furniture and fixtures
Property Plant And Equipment [Line Items]
Total property and equipment	651	51
Leasehold improvements
Property Plant And Equipment [Line Items]
Total property and equipment	578	720
Construction in progress
Property Plant And Equipment [Line Items]
Total property and equipment	$ 45	$ 169

Goodwill and Intangible Assets - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill	$ 949	$ 949
Intangible assets	$ 1,750	$ 1,750

Accrued Liabilities - Summary of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accrued Liabilities Current [Abstract]
Payroll and related expenses	$ 3,971	$ 2,839
Research and clinical expenses	3,909	1,562
Property and equipment	331
Legal and professional expenses	264	296
Other accrued expenses	349	215
Total	$ 8,824	$ 4,912

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 ($) Agreement Target AccountingUnit	Dec. 31, 2016 USD ($)
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		$ 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%
Collaborative Arrangement \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 10,000,000
Collaborative Arrangement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of undetermined targets \| Target	2
Collaborative Arrangement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable	$ 275,000,000
CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue		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]
Deferred revenue		$ 8,900,000

Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)		1 Months Ended	12 Months Ended
	Jul. 07, 2014 USD ($) Target Term ResearchTarget SaleIndicator	Oct. 31, 2015 shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	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 will have additional rights to substitute up to two collaboration targets. 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			$ 15,000,000			$ 10,000,000
Estimated performance period of agreement			10 years
Revised estimated performance period of agreement			7 years
Revenue recognized from collaborative arrangement			$ 9,600,000	$ 5,900,000	$ 2,800,000
Deferred revenue			60,900,000	42,600,000
Amount due from agreement			2,200,000	$ 400,000
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 \| Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received	$ 50,000,000
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
Contingent milestone payments receivable	$ 1,217,000,000
Collaborative Arrangement \| Maximum \| 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 \| Maximum \| 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 \| Maximum \| 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 \| Maximum \| 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 \| Maximum \| 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 \| Maximum \| 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 \| Maximum \| 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
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

Research and Collaboration Agreements - ImmunoGen, Inc - Additional Information (Details) - Collaborative Arrangement - Immuno Gen Inc - USD ($)	1 Months Ended
	Jan. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated total fair value consideration recorded as deferred revenue	$ 13,200,000	$ 13,200,000	$ 13,200,000
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
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
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	1 Months Ended			12 Months Ended
	May 31, 2016	Dec. 31, 2014 USD ($)	May 31, 2013 USD ($) Target SaleIndicator	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of research targets \| Target			2
Nominated number of additional research targets \| Target			2
Deferred revenue				$ 3,400,000	$ 4,900,000
Estimated performance period of agreement			7 years
Additional target as deferred revenue		$ 1,500,000
Adjusted amortization period of deferred revenue	5 years 6 months
Revenue recognized from collaborative arrangement				2,200,000	1,800,000	$ 2,300,000
Amount due from agreement				$ 100,000	$ 400,000
Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 263,500,000
Exercise of License Options [Member] \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			4,500,000
Achievement Of Development Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			38,000,000
Achieving First Commercial Sale In Various Territories \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 101,000,000
Number of sales indicators \| SaleIndicator			3
Achieving Sales Milestones For Each Research Target Program \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable			$ 120,000,000
Up Front Payment Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received			6,000,000
Deferred revenue			$ 6,000,000

License Agreement - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 10, 2015	Dec. 31, 2013
License Agreement [Line Items]
Common stock, shares issued	36,490,169			36,033,209
Annual minimum royalty obligations	$ 150,000
UC Regents
License Agreement [Line Items]
Common stock, shares issued					157,332
Milestone and minimum annual royalty provision paid	$ 2,100,000	$ 300,000	$ 700,000

Long-term Debt - Additional Information (Details) - USD ($)	1 Months Ended
	Oct. 31, 2015	May 31, 2012	Dec. 31, 2016	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)		1 Months Ended	12 Months Ended
	Dec. 10, 2015 USD ($) ft²	Mar. 31, 2016	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Operating Leased Assets [Line Items]
Restricted cash			$ 917,000	$ 917,000
Rent expense			1,800,000	900,000	$ 800,000
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit outstanding, amount			900,000
Restricted cash			$ 900,000	$ 900,000
New Lease Agreement
Operating Leased Assets [Line Items]
Area of rentable office and laboratory space \| ft²	76,173
Lease term description			The term of the New Lease commenced on October 1, 2016. The New 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 New 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
Amendment to Current Lease \| Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date		Jan. 31, 2019
Amendment to Current Lease \| Early Lease Termination
Operating Leased Assets [Line Items]
Lease expiration date		Nov. 30, 2016

Commitments and Contingencies - Schedule of Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2017	$ 3,387
2018	4,374
2019	4,506
2020	4,641
2021 and beyond	29,503
Total	$ 46,411

Convertible Preferred Stock - Additional information (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2015	Dec. 31, 2016	Dec. 31, 2015	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, 2016	Dec. 31, 2015	Dec. 10, 2015	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
Convertible Preferred stock, shares authorized				10,000,000
Convertible 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, 2016	Dec. 31, 2015
Equity [Abstract]
Options issued and outstanding	6,158,746	5,270,751
Shares available for future stock option grants	2,493,188	2,401,406
Total	8,651,934	7,672,157

Stock Option Plans - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	8,651,934	7,672,157
Stock option expiration period	10 years
Aggregate intrinsic value of stock options exercised	$ 4,600,000	$ 2,300,000	$ 1,500
Weighted average grant date fair value, options granted	$ 8.936	$ 7.169	$ 0.945
Unrecognized compensation expense, options granted to employees		$ 23,400,000
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Period of weighted average recognition	2 years 6 months
2015 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	2,493,188	2,444,735
Annual percentage increase in authorized shares in stock plan	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%
2015 Plan \| Vesting per Month After First Anniversary
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted generally vest percent	2.08%
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 Option Plans - Schedule of Activity Under Company's Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Available for Grant
Balances, beginning of the period	2,401,406
Balances, end of the period	2,493,188	2,401,406
Number of Options
Balances, beginning of the period	5,270,751	2,147,872	1,686,490
Options granted	1,356,546	3,309,708	697,557
Options exercised	(414,396)	(173,929)	(6,006)
Options forfeited	(54,155)	(12,900)	(230,169)
Balances, end of the period	6,158,746	5,270,751	2,147,872
Options Exercisable, end of the period	3,071,643	1,908,638
Options vested and expected to vest, end of the period	6,078,576
Options Outstanding, Weighted-Average Exercise Price Per Share
Balances, beginning of the period	$ 3.694	$ 1.197	$ 1.071
Options granted	13.234	5.174	1.449
Options exercised	1.549	1.507	1.260
Options forfeited	4.578	1.405	0.945
Balances, end of the period	5.932	$ 3.694	$ 1.197
Options Exercisable, end of the period	3.750
Options vested and expected to vest, end of the period	$ 5.897
Options Outstanding, Weighted- Average Remaining Contractual Life (years)
Balances, end of the period	8 years
Options Exercisable, end of the period	7 years 3 months 18 days
Options vested and expected to vest, end of the period	8 years
Aggregate Intrinsic Value
Balances, end of the period	$ 34,621
Options Exercisable—December 31, 2016	23,042
Options vested and expected to vest—December 31, 2016	$ 34,351
Employee Stock Option
Options Available for Grant
Balances, beginning of the period	2,401,406	1,896,617	776,628
Options authorized	1,441,328	3,801,597	1,587,377
Options granted	(1,367,546)	(3,309,708)	(697,557)
Options forfeited	18,000	12,900	230,169
Balances, end of the period	2,493,188	2,401,406	1,896,617

Stock Option Plans - Summary of Stock Options Outstanding and Exercisable by Exercise Price (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 3.750
Options, Number Outstanding	6,158,746	5,270,751	2,147,872	1,686,490
Options, Weighted-Average Remaining Contractual Life (Years)	8 years
Options, Number Exercisable	3,071,643	1,908,638
$0.945 - $1.1339
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	742,675
Options, Weighted-Average Remaining Contractual Life (Years)	5 years 3 months 4 days
Options, Number Exercisable	738,002
$0.945 - $1.1339 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 0.945
$0.945 - $1.1339 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.1339
$1.26 - $1.4489
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	622,789
Options, Weighted-Average Remaining Contractual Life (Years)	6 years 7 months 10 days
Options, Number Exercisable	568,230
$1.26 - $1.4489 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.26
$1.26 - $1.4489 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.4489
$1.512 - $1.5119
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	233,999
Options, Weighted-Average Remaining Contractual Life (Years)	7 years 4 months 24 days
Options, Number Exercisable	144,604
$1.512 - $1.5119 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.512
$1.512 - $1.5119 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.5119
$1.575 - $1.5749
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	778,456
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 1 month 10 days
Options, Number Exercisable	499,013
$1.575 - $1.5749 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.575
$1.575 - $1.5749 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 1.5749
$4.473 - $4.4728
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	458,799
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 4 months 6 days
Options, Number Exercisable	205,890
$4.473 - $4.4728 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 4.473
$4.473 - $4.4728 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 4.4728
$6.615 - $6.6147
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	1,940,153
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 7 months 13 days
Options, Number Exercisable	640,426
$6.615 - $6.6147 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 6.615
$6.615 - $6.6147 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 6.6147
$10.17 - $12.28
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Number Outstanding	463,195
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 6 months 4 days
Options, Number Exercisable	61,704
$10.17 - $12.28 \| Minimum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 10.17
$10.17 - $12.28 \| Maximum
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	12.28
$14.06
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 14.06
Options, Number Outstanding	2,000
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 1 month 28 days
$14.46
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 14.46
Options, Number Outstanding	900,246
Options, Weighted-Average Remaining Contractual Life (Years)	9 years 7 days
Options, Number Exercisable	209,089
$21.51
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range	$ 21.51
Options, Number Outstanding	16,434
Options, Weighted-Average Remaining Contractual Life (Years)	8 years 11 months 9 days
Options, Number Exercisable	4,685
$0.945
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 0.945
Options, Number Outstanding		411,576
Options, Weighted-Average Remaining Contractual Life (Years)		7 years 1 month 28 days
Options, Number Exercisable		278,861
$1.134
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.134
Options, Number Outstanding		572,079
Options, Weighted-Average Remaining Contractual Life (Years)		5 years 7 months 10 days
Options, Number Exercisable		572,079
$1.260
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.260
Options, Number Outstanding		342,793
Options, Weighted-Average Remaining Contractual Life (Years)		7 years 18 days
Options, Number Exercisable		306,083
$1.386
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.386
Options, Number Outstanding		164,844
Options, Weighted-Average Remaining Contractual Life (Years)		8 years 18 days
Options, Number Exercisable		148,970
$1.449
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.449
Options, Number Outstanding		249,886
Options, Weighted-Average Remaining Contractual Life (Years)		8 years 3 months 26 days
Options, Number Exercisable		183,100
$1.512
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.512
Options, Number Outstanding		240,058
Options, Weighted-Average Remaining Contractual Life (Years)		8 years 9 months 11 days
Options, Number Exercisable		81,281
$1.575
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 1.575
Options, Number Outstanding		815,030
Options, Weighted-Average Remaining Contractual Life (Years)		9 years 1 month 17 days
Options, Number Exercisable		160,073
$4.473
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 4.473
Options, Number Outstanding		464,799
Options, Weighted-Average Remaining Contractual Life (Years)		9 years 4 months 6 days
Options, Number Exercisable		24,629
$6.615
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 6.615
Options, Number Outstanding		1,966,357
Options, Weighted-Average Remaining Contractual Life (Years)		9 years 7 months 17 days
Options, Number Exercisable		126,667
$12.00
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 12.00
Options, Number Outstanding		26,895
Options, Weighted-Average Remaining Contractual Life (Years)		9 years 9 months 7 days
Options, Number Exercisable		26,895
$21.51
Share Based Compensation Shares Authorized Under Stock Option Plans Exercise Price Range [Line Items]
Options, Exercise Price Range		$ 21.51
Options, Number Outstanding		16,434
Options, Weighted-Average Remaining Contractual Life (Years)		9 years 11 months 12 days

Employee Stock Purchase Plan - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plan Disclosure [Line Items]
Shares available for future purchase employee stock purchase plan	8,651,934	7,672,157
Stock-based compensation expense	$ 10,095,000	$ 3,986,000	$ 553,000
Employee Stock Purchase Plan
Defined Contribution Plan Disclosure [Line Items]
Plan maximum percentage of eligible compensation to purchase shares	15.00%
Plan percentage of fair market value, employee share purchases	85.00%
Number of shares issued during period	31,564
Shares available for future purchase employee stock purchase plan	683,234
Stock-based compensation expense	$ 145,000	$ 0
Unrecognized stock-based compensation cost	$ 113,000

Stock Based Compensation - Total Stock-based Compensation Recorded Related to Options Granted to Employees and Nonemployees and Employee Stock Purchase Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock-based compensation expense:
Stock-based compensation expense	$ 10,095	$ 3,986	$ 553
Research and development
Stock-based compensation expense:
Stock-based compensation expense	4,925	1,972	195
General and administrative
Stock-based compensation expense:
Stock-based compensation expense	$ 5,170	$ 2,014	$ 358

Stock Based Compensation - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 10,095	$ 3,986	$ 553
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	9,400	3,200	500
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 900	$ 800	$ 100

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	76.40%	62.90%	66.40%
Expected volatility, maximum	83.50%	68.90%	71.20%
Risk-free interest rate, minimum	1.20%	1.40%	1.60%
Risk-free interest rate, maximum	2.10%	1.90%	2.00%
Employee Stock Option \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 2 months 12 days	5 years 3 months 18 days
Employee Stock Option \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 10 months 24 days	7 years 2 months 12 days	6 years 1 month 6 days
Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	50.40%
Expected volatility, maximum	75.60%
Risk-free interest rate, minimum	0.50%
Risk-free interest rate, maximum	0.60%
Expected term (in years)	6 months

Related Party Transactions - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Collaborative Arrangement \| Pfizer Inc
Related Party Transaction [Line Items]
Revenue recognized	$ 2,200,000	$ 1,800,000	$ 2,300,000
Deferred revenue	3,400,000	4,900,000
Amount due from agreement	100,000	400,000
Third Rock Ventures
Related Party Transaction [Line Items]
General and administrative expenses from transactions with related party	48,000	33,000	$ 46,000
Accounts payable, related parties	12,000	0
Principal loan amount, outstanding	$ 0	$ 78,000

Income Taxes - Schedule of Components of Provision (Benefit from) for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
State	$ 1	$ 2	$ 1
Total current	1	2	1
Deferred:
Federal	(20)	8	9
Total deferred	(20)	8	9
Provision for (benefit from) income taxes	$ (19)	$ 10	$ 10

Income Taxes - Schedule of Effective Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	0.80%	0.80%	1.00%
Stock compensation	(0.60%)	(1.10%)	(0.30%)
Tax attributes subject to 382 limitation	0.00%	(35.40%)	0.00%
Tax credits	2.20%	0.80%	1.30%
Other	(0.80%)	(1.80%)	(0.10%)
Change in valuation allowance	(35.60%)	2.70%	(35.90%)

Income Taxes - Schedule of Deferred Income Tax Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components Of Deferred Tax Assets And Liabilities [Abstract]
Net operating loss carryforwards	$ 24,528	$ 5,688	$ 22,484
Research and development credits	2,683	1,337	2,009
Intangible—in-process R&D	81	88	96
Deferred revenue	13,857	16,182	1,579
Accruals and deferred rent	1,335	998	276
Stock-based compensation	3,963	1,125	155
Other	1	26
Total gross deferred income tax assets	46,448	25,444	26,599
Less: valuation allowance	(46,137)	(25,043)	(26,012)
Deferred tax assets, net of valuation allowance	311	401	587
Fixed assets	(229)	(313)	(491)
In-process R&D	(595)	(595)	(595)
Deferred tax liabilities	(824)	(908)	(1,086)
Net deferred income tax liabilities	$ (513)	$ (507)	$ (499)

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Contingency [Line Items]
Deferred tax asset, net change in total valuation allowance increase (decrease) during period	$ 21,100,000	$ (1,000,000)	$ 10,900,000
Unrecognized tax benefits	1,182,000	666,000	3,019,000	$ 532,000
Unrecognized tax benefits which would affect company's effective tax rate due to valuation allowance	0
Interest and penalties accrued	$ 0	$ 0	$ 0
Minimum
Income Tax Contingency [Line Items]
Percentage of change in ownership	50.00%
Federal
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 71,500,000
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	2016
Federal \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 1,700,000
Tax credit carryforward, expiration	2030
State
Income Tax Contingency [Line Items]
Net operating loss carryforwards	$ 14,300,000
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	2016
State \| Research and Development Tax Credits
Income Tax Contingency [Line Items]
Reduction to tax credit carryforwards	$ 2,900,000

Income Taxes- Schedule of Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at the beginning of the year	$ 666	$ 3,019	$ 532
Additions based on tax positions related to current year	23	(2,312)	2,473
Adjustment based on submitted prior year tax returns	493	(41)	14
Balance at end of the year	$ 1,182	$ 666	$ 3,019

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net loss per share	6,086,939	21,630,281	17,338,185
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	15,024,251
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	244,782
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	81,620
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	6,086,939	3,865,842	1,987,532

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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (14,033)	$ (14,662)	$ (14,176)	$ (16,029)	$ (11,487)	$ (11,870)	$ (5,806)	$ (6,211)	$ (58,900)	$ (35,374)	$ (30,310)
Accretion to redemption value and cumulative dividends on preferred stock										(6,705)	(4,566)
Net loss attributable to common stockholders	$ (14,033)	$ (14,662)	$ (14,176)	$ (16,029)	$ (12,045)	$ (14,828)	$ (7,563)	$ (7,643)	$ (58,900)	$ (42,079)	$ (34,876)
Denominator:
Weighted-average common shares outstanding used to calculate net loss per share attributable to common stockholders, basic and diluted									36,234,732	8,595,247	989,453
Net loss per share attributable to common stockholders, basic and diluted									$ (1.63)	$ (4.90)	$ (35.25)

Supplementary Data - Quarterly Financial Data (Unaudited) - Unaudited Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 6,272	$ 3,454	$ 3,094	$ 2,223	$ 1,988	$ 1,939	$ 2,043	$ 1,742	$ 15,043	$ 7,712	$ 5,077
Net loss	(14,033)	(14,662)	(14,176)	(16,029)	(11,487)	(11,870)	(5,806)	(6,211)	(58,900)	(35,374)	(30,310)
Net loss attributable to common stockholders	$ (14,033)	$ (14,662)	$ (14,176)	$ (16,029)	$ (12,045)	$ (14,828)	$ (7,563)	$ (7,643)	$ (58,900)	$ (42,079)	$ (34,876)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.39)	$ (0.40)	$ (0.39)	$ (0.44)	$ (0.39)	$ (14.26)	$ (7.56)	$ (7.67)	$ (1.63)	$ (4.90)	$ (35.25)

Research and Collaboration Agreements - MD Anderson - Additional Information (Details)	Nov. 02, 2015
MD Anderson
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option expiration date	Nov. 02, 2019