Five Prime Therapeutics (FPRX) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 19, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	FPRX
Entity Registrant Name	FIVE PRIME THERAPEUTICS INC
Entity Central Index Key	1,175,505
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Shell Company	false
Entity Emerging Growth Company	false
Entity Small Business	true
Entity Common Stock, Shares Outstanding		35,487,149
Entity Public Float			$ 549

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 43,953	$ 59,790
Marketable securities	226,185	232,900
Receivables from collaborative partners	5,096	13,133
Prepaid and other current assets	13,334	5,367
Total current assets	288,568	311,190
Restricted cash	1,543	1,543
Property and equipment, net	28,718	30,762
Other long-term assets	2,705	552
Total assets	321,534	344,047
Current liabilities:
Accounts payable	1,972	2,237
Accrued personnel-related expenses	7,383	7,156
Other accrued liabilities	15,348	27,519
Deferred revenue, current portion	1,428	12,713
Deferred rent, current portion	1,356	1,356
Total current liabilities	27,487	50,981
Deferred revenue, long-term portion	10,465	10,223
Deferred rent, long-term portion	18,443	17,641
Commitments and contingencies (Note 11)
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized, 35,625,751 issued and 34,745,721 outstanding at December 31, 2018. 28,982,056 issued and 28,178,639 outstanding at December 31, 2017.	34	28
Preferred stock, $0.001 par value, 10,000,000 shares authorized; no shares issued and outstanding
Additional paid-in capital	559,892	421,257
Accumulated other comprehensive loss	(106)	(476)
Accumulated deficit	(294,681)	(155,607)
Total stockholders’ equity	265,139	265,202
Total liabilities and stockholders’ equity	$ 321,534	$ 344,047

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	35,625,751	28,982,056
Common stock, shares outstanding	34,745,721	28,178,639
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Statements of Operations

Statements of Operations - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration and license revenue	$ 49,868,000	$ 39,508,000	$ 30,691,000
Type of Revenue [Extensible List]	fprx:CollaborationMember	fprx:CollaborationMember	fprx:CollaborationMember
Operating expenses:
Research and development	$ 156,352,000	$ 150,908,000	$ 94,072,000
General and administrative	39,671,000	40,002,000	35,831,000
Total operating expenses	196,023,000	190,910,000	129,903,000
Loss from operations	(146,155,000)	(151,402,000)	(99,212,000)
Interest income	5,792,000	2,978,000	2,467,000
Other loss, net	(84,000)	(94,000)
Loss before income tax	(140,447,000)	(148,518,000)	(96,745,000)
Income tax (provision) benefit	0	(1,704,000)	31,048,000
Net loss	$ (140,447,000)	$ (150,222,000)	$ (65,697,000)
Basic and diluted net loss per common share	$ (4.13)	$ (5.38)	$ (2.44)
Weighted-average shares used to compute basic and diluted net loss per common share	33,976	27,945	26,955

Statements of Comprehensive Los

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (140,447)	$ (150,222)	$ (65,697)
Other comprehensive gain (loss):
Unrealized gain (loss) on marketable securities, net of tax	370	(437)	35
Comprehensive loss	$ (140,077)	$ (150,659)	$ (65,662)

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Beginning Balance at Dec. 31, 2015	$ 433,206	$ 26	$ 372,605	$ (74)	$ 60,649
Beginning Balance, Shares at Dec. 31, 2015		26,116,886
Issuance of common stock under equity incentive plans and related excess tax benefits	5,200	$ 1	5,199
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares		1,730,340
Repurchase of shares to satisfy tax withholding obligations	(14,054)		(14,054)
Repurchase of shares to satisfy tax withholding obligations, Shares		(338,149)
Stock-based compensation expense	32,885		32,885
Other comprehensive gain (loss)	35			35
Net loss	(65,697)				(65,697)
Ending Balance at Dec. 31, 2016	391,575	$ 27	396,635	(39)	(5,048)
Ending Balance, Shares at Dec. 31, 2016		27,509,077
Issuance of common stock under equity incentive plans and related excess tax benefits	4,022	$ 1	4,021
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares		992,556
Repurchase of shares to satisfy tax withholding obligations	(13,909)		(13,909)
Repurchase of shares to satisfy tax withholding obligations, Shares		(322,994)
Cumulative effect of adoption of ASU | ASU 2016-09 [Member]			337		(337)
Stock-based compensation expense	34,173		34,173
Other comprehensive gain (loss)	(437)			(437)
Net loss	(150,222)				(150,222)
Ending Balance at Dec. 31, 2017	$ 265,202	$ 28	421,257	(476)	(155,607)
Ending Balance, Shares at Dec. 31, 2017	28,178,639	28,178,639
Issuance of common stock upon follow-on public offering, netof issuance costs	$ 115,000	$ 6	114,994
Issuance of stock, Shares		5,897,435
Issuance costs related to the follow-on public offering	(7,388)		(7,388)
Issuance of common stock under equity incentive plans and related excess tax benefits	3,963		3,963
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares		821,456
Repurchase of shares to satisfy tax withholding obligations	(2,402)		(2,402)
Repurchase of shares to satisfy tax withholding obligations, Shares		(151,809)
Cumulative effect of adoption of ASU | ASU 2014-09 [Member]	1,373				1,373
Stock-based compensation expense	29,468		29,468
Other comprehensive gain (loss)	370			370
Net loss	(140,447)				(140,447)
Ending Balance at Dec. 31, 2018	$ 265,139	$ 34	$ 559,892	$ (106)	$ (294,681)
Ending Balance, Shares at Dec. 31, 2018	34,745,721	34,745,721

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net income loss	$ (140,447)	$ (150,222)	$ (65,697)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,020	2,513	1,742
Loss on disposal of property and equipment	38	95	9
Stock-based compensation expense	29,468	34,173	32,885
Amortization of discounts and premiums on marketable securities	(1,750)	1,621	4,187
Excess tax benefits from employee equity incentive plans			3,123
Deferred income taxes			15,071
Changes in operating assets and liabilities:
Receivables from collaborative partners	8,037	(9,174)	95
Income tax receivable		4,670	(4,670)
Prepaid, other current assets, and other long-term assets	(10,120)	4,235	(2,999)
Restricted cash			(1,543)
Accounts payable	(265)	1,903	(1,560)
Accrued personnel-related expenses	227	(801)	1,079
Deferred revenue	(9,670)	(9,070)	(16,771)
Deferred rent	802	3,699	(768)
Income tax payable			(52,843)
Other accrued liabilities, and other long-term liabilities	(3,854)	4,174	8,909
Net cash used in operating activities	(122,514)	(112,184)	(79,751)
Investing activities
Purchases of marketable securities	(377,365)	(330,363)	(516,752)
Maturities of marketable securities	386,200	509,500	466,000
Proceeds from disposal of property and equipment		12
Purchases of property and equipment	(11,331)	(4,941)	(2,961)
Net cash (used in) provided by investing activities	(2,496)	174,208	(53,713)
Financing activities
Proceeds from public offering of common stock, net of issuance costs	107,612
Proceeds from issuance of common stock under equity incentive plans	3,963	4,022	8,323
Repurchase of shares to satisfy tax withholding obligations	(2,402)	(13,909)	(14,054)
Excess tax benefits from employee equity incentive plans			(3,123)
Net cash provided by (used in) financing activities	109,173	(9,887)	(8,854)
Net (decrease) increase in cash and cash equivalents and restricted cash	(15,837)	52,137	(142,318)
Cash, cash equivalents and restricted cash at beginning of period	61,333	9,196	149,971
Cash, cash equivalents and restricted cash at end of period	45,496	61,333	9,196
Supplemental disclosure
Income taxes paid		1,704	11,433
Property and equipment purchases included in accrued liabilities	716	9,033	1,232
Tenant improvement by the landlord		14,324
Supplemental cash flow information
Cash and cash equivalents at beginning of period	59,790	7,653	149,971
Restricted cash at beginning of period	1,543	1,543
Cash and cash equivalents at end of period	43,953	59,790	7,653
Restricted cash at end of period	$ 1,543	$ 1,543	$ 1,543

Business

Business	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Business	1. Business Five Prime Therapeutics, Inc. (we, us, our, or the Company) is a clinical-stage biotechnology company focused on discovering and developing innovative protein therapeutics. We were incorporated in December 2001 in Delaware. Our operations are based in South San Francisco, California and we operate in one segment. We have reclassified certain prior period amounts within our footnotes to conform to our current period presentation.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes as of the date of the financial statements. The most significant estimates in the Company’s financial statements include the recognition of revenue, stock-based compensation, completeness of clinical trial accruals and income taxes. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable. Cash and Cash Equivalents We consider all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents are recorded at face value, or cost, which approximates fair value. Restricted Cash Restricted cash consists of a certificate of deposit held by our bank as collateral for a standby letter of credit in the same notional amount by our landlord to secure our obligations under our corporate office and laboratory facility lease entered in December 2016. We are required to maintain this restricted cash balance, the amount of which is subject to reduction starting on January 1, 2023, if certain conditions are met, for the duration of this lease Marketable Securities All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. We consider our available-for-sale portfolio as available for use in current operations. Accordingly, we classify certain investments as short-term marketable securities, even though the stated maturity date may be one year or more beyond the current balance sheet date. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive income or loss and reported as a separate component of stockholders’ equity or deficit until realized. Realized gains and losses and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. We adjust the amortized cost of securities for amortization of premiums and accretion of discounts to maturity. We include interest on short-term investments in interest income. In accordance with our investment policy, management invests to diversify credit risk and only invests in debt securities with high credit quality, including U.S. government securities. We periodically evaluate whether declines in the fair value of our investments below their cost are other than temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities, and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. If we determine that the decline in fair value of an investment is below its accounting basis and this decline is other than temporary, we would reduce the carrying value of the security we hold and record a loss for the amount of such decline. We have not recorded any realized losses or declines in value judged to be other than temporary on our investments in debt securities. Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash and cash equivalents and marketable securities. Cash and cash equivalents and marketable securities are invested through banks and other financial institutions in the United States. Such deposits in the United States may be in excess of insured limits. Fair Value of Financial Instruments We determine the fair value of financial and nonfinancial assets and liabilities using the fair value hierarchy, which describes three levels of inputs that may be used to measure fair value, as follows: Level 1 —Quoted prices in active markets for identical assets or liabilities; Level 2 —Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices for identical or similar assets or liabilities 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. For our marketable securities, we review trading activity and pricing as of the measurement date. When sufficient quoted pricing for identical securities is not available, we use market pricing and other observable market inputs for similar securities obtained from various third-party data providers. These inputs either represent quoted prices for similar assets in active markets or have been derived from observable market data; and Level 3 —Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, obtained from various third-party data providers, represent quoted prices for similar assets in active markets and were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data. There were no transfers between Level 1 and Level 2 securities in the periods presented. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. We do not have any assets or liabilities measured using Level 3 inputs as of December 31, 2018. The following table summarizes our financial instruments that were measured at fair value on a recurring basis by level of input within the fair value hierarchy defined above (in thousands): December 31, 2018 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 40,849 $ 40,849 $ — $ — U.S. Treasury securities 104,140 104,140 — — Agency bonds 53,999 53,999 — — Corporate bonds 11,893 — 11,893 — Commercial paper 56,152 — 56,152 — Certificate of deposit 1,543 — 1,543 — Total $ 268,576 $ 198,988 $ 69,588 $ — December 31, 2017 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 31,802 $ 31,802 $ — $ — U.S. Treasury securities 232,900 232,900 — — Certificate of deposit 1,543 — 1,543 — Total $ 266,245 $ 264,702 $ 1,543 $ — Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term. Impairment of Long-Lived Assets Long-lived assets include property and equipment. We review the carrying value of long-lived assets for impairment whenever events or changes in circumstances indicate that the assets may not be recoverable. We recognize an impairment loss when the total estimated future cash flows expected to result from the use of the asset and its eventual disposition are less than the carrying amount. Through December 31, 2018, there have been no such impairment losses. Revenue Recognition Effective January 1, 2018, we adopted Financial Accounting Standards Board, or FASB, Accounting Standard Update, or ASU , Revenue from Contracts with Customers (Topic 606) The terms of our collaborative research and development agreements include upfront and license fees, research, development and other funding or reimbursements, milestone and other contingent payments for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of commercialized products. Arrangements that include upfront payments may require deferral of revenue recognition to a future period until we perform obligations under these arrangements. We record research and development funding payable to us as accounts receivable when our right to consideration is unconditional. The event-based milestone and other contingent payments represent variable consideration, and we use the most likely amount method to estimate this variable consideration. Given the high degree of uncertainty around occurrence of these events, we determine the milestone and other contingent amounts to be fully constrained until the uncertainty associated with these payments is resolved. We will recognize revenue from sales-based royalty payments when or as the sales occur. We will re-evaluate the transaction price in each reporting period as uncertain events are resolved and other changes in circumstances occur. A performance obligation is a promise in a contract to transfer a distinct good or service and is the unit of accounting in Topic 606. A contract’s transaction price is allocated among each distinct performance obligation based on relative standalone selling price and recognized as revenue when, or as, the applicable performance obligation is satisfied. Under Topic 606, we elected to use the practical expedient permitted related to adoption, which does not require us to disclose certain information regarding our remaining performance obligations as of the end of the reporting period prior to the initial date of adoption. Additionally, we elected the practical expedient for certain research and development funding which allows us to recognize revenue in the amount for which we have a right to invoice if our right to consideration is an amount that corresponds directly to the value of our performance completed to date. As a result, we effectively bypass the steps of determining the transaction price and allocating that transaction price to the performance obligation. Research and Development Expenses Research and development expenses consist of costs we incur for our own and for sponsored and collaborative research and development activities. Research and development costs are expensed as incurred. Research and development costs consist of salaries and benefits, including associated stock-based compensation, laboratory supplies and facility costs, as well as fees paid to other entities that conduct certain research and development activities on our behalf. We estimate preclinical study and clinical trial expenses based on the services performed pursuant to contracts with research institutions and contract research organizations, or CROs, and clinical manufacturing organizations, or CMOs, that conduct and manage preclinical studies and clinical trials on our behalf based on actual time and expenses incurred by them. Further, we accrue expenses related to clinical trials based on the level of patient activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. If we do not identify costs that we have begun to incur or if we underestimate or overestimate the level of services performed or the costs of these services, our actual expenses could differ from our estimates. We expense payments for the acquisition and development of technology as research and development costs if, at the time of payment, the technology: is under development; is not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing; has not reached technical feasibility; or otherwise has no foreseeable alternative future use. Stock-Based Compensation We recognize compensation expense using a fair-value-based method for costs related to all share-based payments, including restricted stock awards, or RSAs, and stock option awards. For RSAs, stock-based compensation cost is based on the closing market value of our common stock at the date of grant and is recognized as expense ratably over the requisite service period. For stock option awards, stock-based compensation cost is measured at the grant date, based on the fair-value-based measurement of the award estimated using the Black-Scholes option-pricing model, and is recognized as expense over the requisite service period on a straight-line basis. We account for forfeitures as they occur by reversing any expense recognized for unvested awards. Income Taxes We account for income taxes using the liability method, under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided when the expected realization of the deferred tax assets does not meet the more-likely-than-not criteria. As a result, deferred tax assets at the end of 2018 and 2017 are subject to a full valuation allowance. We are required to determine whether it is more likely than not that a tax position will be sustained upon examination by the appropriate taxing authorities before any part of the benefit can be recorded in the financial statements. It is our practice to recognize interest and penalties related to unrecognized tax benefits, if any, as a component of income tax expense. Accounting Pronouncements Adopted in 2018 In May 2014, FASB issued Topic 606, which supersedes nearly all existing revenue recognition guidance under U.S. generally accepted accounting principles , or GAAP. FASB subsequently issued amendments to Topic 606 that have the same effective date and transition date. The core principle of Topic 606 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. Topic 606 defines a five-step process to achieve this core principle and, in as a result, more judgment and estimates may be required in the course of the revenue recognition process, including with respect to identifying performance obligations in a contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. We adopted Topic 606, effective January 1, 2018, using the modified retrospective transition method, in which the new standard is applied as of the date of initial adoption. We applied the standard to contracts that were not completed at the date of initial application. We recorded the cumulative effect of initially applying the standard as an adjustment to the opening balance of retained earnings. The adoption of the new revenue recognition guidance resulted in a decrease of $1.4 million to deferred revenue and an increase of $1.4 million to retained earnings as of January 1, 2018. Additionally, we determined that the classification between deferred revenue, current portion, and deferred revenue, long-term portion, changed as a result of adoption of Topic 606. We concluded that we will classify deferred revenue for all licensing and collaboration arrangements as deferred revenue, long-term portion, and will reclassify to deferred revenue, current portion, when the remaining term of the estimated performance period is one year or less. Our adoption of Topic 606 effective January 1, 2018 affected the following financial statement line items: Condensed Statements of Operations Year Ended December 31, 2018 (in thousands, except per share data) Under Topic 606 Under Topic 605 Effect of change Collaboration and license revenue $ 49,868 $ 52,329 $ (2,461 ) Operating expenses 196,023 196,023 — Operating loss $ (146,155 ) $ (143,694 ) $ (2,461 ) Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Net loss per share applicable to common stockholders - basic and diluted $ (4.13 ) $ (4.06 ) $ (0.07 ) Condensed Balance Sheets December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Receivables from collaborative partner $ 5,096 $ 5,096 $ — Deferred revenue, current portion 1,428 8,187 (6,759 ) Deferred revenue, long-term portion 10,465 2,618 7,847 Accumulated deficit (294,681 ) (293,593 ) (1,088 ) Condensed Statement of Cash Flows Year Ended December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Decrease in deferred revenue in connection with Topic 606 adoption 1,373 — 1,373 Changes in operating assets and liabilities Receivables from collaborative partner 8,037 8,037 — Deferred revenue (11,043 ) (12,131 ) 1,088 Cash, cash equivalents and restricted cash at beginning of period 61,333 61,333 — Cash, cash equivalents and restricted cash at end of period 45,496 45,496 — In May 2017, FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718) – Scope of Modification Accounting In November 2016, FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) – Restricted Cash In June 2018, FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718) – Improvements to Nonemployee Share-Based Payment Accounting Subtopic 505-50, Equity–Equity-Based Payments to Non-Employees Accounting Pronouncements Not Yet Adopted In November 2018, FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808) clarifies when certain transactions between collaborative arrangement participants should be accounted for under Topic 606 and incorporates unit-of-account guidance consistent with Topic 606 to aid in this determination. ASU 2018-18 will become effective January 1, 2020 and will apply to all annual and interim reporting periods thereafter. Early adoption is permitted. ASU 2018-18 should generally be applied retrospectively to the date of initial application of Topic 606. We do not anticipate that the adoption of this standard will have a material effect on our financial statements In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. These amendments eliminate, modify, or integrate into other SEC requirements certain disclosure rules. Among the amendments is the requirement to present an analysis of changes in stockholders’ equity in the interim financial statements included in quarterly reports on Form 10-Q. The analysis, which can be presented as a footnote or separate statement, is required for the current and comparative quarter and year-to-date interim periods. The amendments are effective for all filings made on or after November 5, 2018. In light of the anticipated timing of effectiveness of the amendments and expected proximity of effectiveness to the filing date for most filers’ quarterly reports, the SEC’s Division of Corporate Finance issued a Compliance and Disclosure Interpretation related to Exchange Act Forms, or CDI – Question 105.09, that provides transition guidance related to this disclosure requirement. CDI – Question 105.09 states that the SEC would not object if the filer’s first presentation of the changes in shareholders’ equity is included in its quarterly report on Form 10-Q for the quarter that begins after the effective date of the amendments. As such, we adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity beginning the first quarter of 2019. We do not anticipate that the adoption of these SEC amendments will have a material effect on our financial position, results of operations, cash flows or shareholders’ equity. In August 2018, FASB issued ASU 2018-13, Fair Value Measurement - Disclosure Framework (Topic 820) In June 2016, FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326) In February 2016, FASB issued ASU 2016-02, Leases (Topic 842)

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Cash Equivalents and Marketable Securities	3. Cash Equivalents and Marketable Securities The following is a summary of our cash equivalents and marketable securities at December 31, 2018 and 2017 (in thousands): December 31, 2018 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 40,849 $ — $ — $ 40,849 U.S. Treasury securities 104,218 — (78 ) 104,140 Agency bonds 54,005 9 (15 ) 53,999 Corporate bonds 11,897 — (4 ) 11,893 Commercial paper 56,171 0 (19 ) 56,152 Total cash equivalents and marketable securities 267,140 9 (115 ) 267,034 Less: cash equivalents (40,849 ) — — (40,849 ) Total marketable securities $ 226,291 $ 9 $ (115 ) $ 226,185 December 31, 2017 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 31,802 $ — $ — $ 31,802 U.S. Treasury securities 233,376 — (476 ) 232,900 Total cash equivalents and marketable securities 265,178 — (476 ) 264,702 Less: cash equivalents (31,802 ) — — (31,802 ) Total marketable securities $ 233,376 $ — $ (476 ) $ 232,900 As of December 31, 2018, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands): Estimated Amortized Fair Cost Value Debt securities maturing: In one year or less $ 226,291 $ 226,185 Total marketable securities $ 226,291 $ 226,185 Our cash equivalents and marketable securities have an average maturity of approximately four months and the longest maturity is ten months. There have been no significant realized gains or losses on our available-for-sale securities for the periods presented. We determined that the gross unrealized losses of $0.1 million on our marketable securities as of December 31, 2018 were temporary in nature and related primarily to interest rate shifts rather than significant changes in the underlying credit quality of the securities that we hold. We currently do not intend to sell these securities prior to maturity and do not consider these investments to be other-than-temporarily impaired at December 31, 2018. There were no sales of available-for-sale securities in any of the periods presented.

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and Equipment Property and equipment consist of the following (in thousands): December 31, 2018 2017 Computer equipment and software $ 2,403 $ 1,892 Furniture and fixtures 968 947 Laboratory equipment 19,579 17,429 Leasehold improvements 22,175 22,175 $ 45,125 $ 42,443 Less: accumulated depreciation and amortization (16,407 ) (11,681 ) Property and equipment, net $ 28,718 $ 30,762 We entered into a lease agreement with respect to our new corporate office and laboratory facility in December 2016. During fiscal 2017, we acquired $22.2 million of leasehold improvements in connection with our move to the new office. We received lease incentives totaling $14.4 million from our landlord for a portion of the costs of these leasehold improvements.

Other Accrued Liabilities

Other Accrued Liabilities	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Other Accrued Liabilities	5. Other Accrued Liabilities Other accrued liabilities consist of the following (in thousands): December 31, 2018 2017 Clinical development $ 10,513 $ 12,580 Manufacturing 1,104 2,835 Trade payable 3,381 3,995 Unpaid leasehold improvements — 7,742 Other 350 367 Total accrued liabilities $ 15,348 $ 27,519

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity	6. Stockholders’ Equity We have 110,000,000 shares of authorized capital stock issuable in series, all with a par value of $0.001 per share, of which 100,000,000 shares are designated as common stock and 10,000,000 shares are designated as preferred stock. Our Board is authorized to determine the designation, powers, preferences and rights of any such series. As of December 31, 2018 and 2017, we had 34,745,721 and 28,178,639 shares of common stock outstanding, respectively. There were no shares of preferred stock outstanding as of December 31, 2018 and 2017. In January 2018, we closed on a public offering of 5,897,435 shares of our common stock, which included 769,230 shares sold upon the underwriters' full exercise of their option to purchase additional shares, resulting in aggregate gross proceeds of $115.0 million, before deducting underwriting discounts and commissions and estimated offering expenses payable by us, and net proceeds of approximately $107.6 million after deducting these amounts Equity Incentive Plans Our Board of Directors, or Board, and stockholders previously approved the 2002 Equity Incentive Plan, or the 2002 Plan, and the 2010 Equity Incentive Plan, or the 2010 Plan, and collectively with the 2002 Plan, the Prior Plans. The 2002 Plan terminated in March 2012. In September 2013, our stockholders approved the 2013 Omnibus Incentive Plan, or the 2013 Plan. As of September 23, 2013, the effective date of the 2013 Plan, we suspended the 2010 Plan and no additional awards may be granted under the 2010 Plan. Any shares of common stock covered by awards granted under the Prior Plans that terminate after September 23, 2013 by expiration, forfeiture, cancellation or other means without the issuance of such shares were added to the 2013 Plan reserve. The initial number of shares of common stock available for issuance under the 2013 Plan was 3,500,000, which includes the 1,069,985 shares of common stock that were available for issuance under the Prior Plans as of the effective date of the 2013 Plan. Unless our Board provides otherwise, beginning on January 1, 2014 and continuing until the expiration of the 2013 Plan, the total number of shares of common stock available for issuance under the 2013 Plan will automatically increase annually on January 1 by 4% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year. Under the plan, any shares that are forfeited or expired are added back to the shares available for issuance. As of December 31, 2018, 1,884,387 shares of common stock were available for future issuance of options, restricted stock and other stock-based awards under the 2013 Plan. Incentive stock options may be granted with an exercise price of not less than estimated fair value. Stock options granted to a stockholder owning more than 10% of our voting stock must have an exercise price of not less than 110% of the estimated fair value of the common stock on the date of grant. For all stock options granted prior to our initial public offering, our Board determined the estimated fair value of our common stock. For all stock options granted after the completion of our initial public offering in September 2013, the fair value for our underlying common stock is determined using the closing market price on the date of grant. Stock options are granted with terms of up to ten years and generally vest over a period of four years. The following table summarizes option activity under our stock plans and related information: Options Outstanding Weighted- Weighted- Average Average Exercise Remaining Aggregate Number Price Contractual Intrinsic of Shares Per Share Terms Value (in years) (in thousands) Balance at January 1, 2018 3,867,645 $ 30.35 Options granted 858,100 17.05 Options exercised (328,585 ) 8.18 Options forfeited (303,640 ) 34.20 Options expired (383,339 ) 35.69 Balance at December 31, 2018 3,710,181 28.37 7.05 $ 740,587 Options exercisable at December 31, 2018 2,195,470 26.95 6.03 740,587 The weighted-average grant-date fair value per share of stock options granted during the years ended December 31, 2018, 2017 and 2016 was $10.85, $25.78 and $27.95 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2018, 2017 and 2016 was $2.4 million, $5.4 million and $30.8 million, respectively. We recorded stock-based compensation expense related to options of approximately $17.3 million, $19.7 million and $11.4 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, there was $27.9 million of total unrecognized compensation expense that we expect to recognize over a weighted-average period of 2.5 years. RSAs are share awards that entitle the holder to receive freely tradable shares of our common stock upon vesting and are unforfeitable once fully vested. The fair value of RSAs was based upon the closing sales price of our common stock on the grant date. The following table summarizes the RSA activity under our stock plans and related information: RSAs Outstanding Weighted-Average Number Grant-Date of Shares Fair Value Unvested balance at January 1, 2018 803,417 $ 40.24 RSAs granted 715,775 17.62 RSAs vested (392,136 ) 34.89 RSAs forfeited (247,026 ) 32.66 Unvested balance at December 31, 2018 880,030 26.36 The total fair value on the date of vesting of RSAs that vested in 2018, 2017 and 2016 was $6.2 million, $30.8 million, and $33.2 million, respectively. We recorded stock-based compensation expense related to RSAs of approximately $11.6 million, $14.0 million and $20.9 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, there was $16.6 million of unrecognized compensation cost related to unvested employee and director RSAs that we expect to recognize over a weighted-average period of 1.8 years. Employee Stock Purchase Plan In September 2013, our stockholders approved the 2013 Employee Stock Purchase Plan, or the ESPP, which became effective as of September 23, 2013. We initially reserved a total of 250,000 shares of common stock for issuance under the ESPP. Unless our Board provides otherwise, continuing until the expiration of the ESPP, the total number of shares of common stock available for issuance under the ESPP will automatically increase annually on January 1 by the lesser of (i) 1% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year, or (ii) 300,000 shares of common stock. As of December 31, 2018, 1,138,877 shares of common stock were available for issuance under the ESPP. Under our ESPP, employees can purchase shares of our common stock based on a percentage of their compensation subject to certain limits. The purchase price per share is equal to the lower of 85% of the fair market value of our common stock on the offering date or the purchase date with a six-month look-back feature. ESPP purchases are settled with common stock from the ESPP’s previously authorized and available pool of shares. We issued a total of 100,735 shares under the ESPP in 2018. The compensation expense related to the ESPP was $0.5 million, $0.5 million and $0.6 million for the years ended December 31, 2018, 2017 and 2016, respectively. As of December 31, 2018, there was $0.2 million of unrecognized compensation cost related to the ESPP, which we expect to recognize over 4.4 months. Stock-Based Compensation Total stock-based compensation expense recognized was as follows: Year Ended December 31, (in thousands) 2018 2017 2016 Research and development $ 15,426 $ 18,285 $ 17,960 General and administrative 14,042 15,888 14,925 Total $ 29,468 $ 34,173 $ 32,885 We estimated the fair value of each award using the Black-Scholes option-pricing model based on the date of grant of such award with the following assumptions: Options ESPP Year Ended December 31, Year Ended December 31, 2018 2017 2016 2018 2017 2016 Expected term (years) 5.5-6.3 5.5-6.3 5.5-6.3 0.5 0.5 0.5 Expected volatility 68-70% 66-70% 69-74% 47-94% 42-94% 47-57% Risk-free interest rate 2.6-2.9% 1.9-2.2% 1.3-1.8% 1.4-2.5% 1.0-1.4% 0.4-0.6% Expected dividend yield 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% The expected term of options granted represents the period of time that we expect options granted to remain outstanding, which we determined using the simplified method as we have insufficient historical information to provide a basis for estimate. The expected term of the ESPP rights is equal to the six-month look-back period. Volatility for options granted is based on the historical volatility of our stock price since we became publicly traded. Volatility for ESPP rights is equal to our historical volatility over the six-month look-back period. The risk-free interest rate for the expected term of the options is based on the U.S. Treasury yield curve with a maturity equal to the expected term in effect at the time of grant. We have not paid, and do not anticipate paying, cash dividends on our shares of common stock; therefore, the expected dividend yield is zero.

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Earnings per Share	7. Earnings per Share The computation of basic loss per share is based on the weighted-average number of our common shares outstanding. The computation of diluted loss per share is based on the weighted-average number of our common shares outstanding and dilutive potential common shares, which include shares that may be issued under our equity incentive plans, determined using the treasury stock method. The following table sets forth the computation of basic and diluted net loss (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (140,447 ) $ (150,222 ) $ (65,697 ) Denominator: Denominator for basic loss per share - weighted-average shares 33,976 27,945 26,955 Denominator for diluted loss per share 33,976 27,945 26,955 Basic and diluted net loss per share $ (4.13 ) $ (5.38 ) $ (2.44 ) We excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands): Year Ended December 31, 2018 2017 2016 Options to purchase common stock 3,710 3,843 2,981 RSAs 880 886 1,278 Total 4,590 4,729 4,259

License and Collaboration Agree

License and Collaboration Agreements	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
License and Collaboration Agreements	8. License and Collaboration Agreements The following table presents changes during the year ended December 31, 2018 in the balances of our contract assets, including receivables from collaboration partners, and contract liabilities, including deferred revenue. (in thousands) Contract Assets Balance at January 1, 2018 $ 13,133 Additions 39,262 Deductions (47,299 ) Balance at December 31, 2018 $ 5,096 (in thousands) Contract Liabilities Balance at January 1, 2018 $ 21,563 Additions for advance billings 4,727 Deductions for performance obligations satisfied in current period (12,218 ) Deductions for performance obligations satisfied in the prior periods in connection with updates to the measure of progress (2,179 ) Balance at December 31, 2018 $ 11,893 Bristol-Myers Squibb Company Immuno-Oncology Research Collaboration In March 2014, we entered into a research collaboration and license agreement, or the Immuno-Oncology Research Collaboration, with Bristol-Myers Squibb Company, or BMS We received an upfront payment of $20.0 million from BMS in April 2014 in connection with our entry into the Immuno-Oncology Research Collaboration. BMS was obligated to pay us $9.5 million in research funding over the course of the three-year research term based on the research activities currently planned under the research plan. BMS had the option to extend the research term for two additional one-year periods on a year-by-year basis for an additional $2.1 million for each extension, during which extensions we would be obligated to perform additional services as agreed to with BMS and BMS would be obligated to pay us research funding with respect to such services. The initial research term under the Immuno-Oncology Research Collaboration expired in March 2017. In each of December 2016 and December 2017, BMS exercised its option to extend the research term for an additional year to March 2018 and March 2019, respectively. In connection with entering into the Immuno-Oncology Research Collaboration, BMS purchased 994,352 shares of our common stock at a price per share of $21.16, for an aggregate purchase price of $21.0 million. We determined that the purchase price of $21.16 per share exceeded the fair value of our common stock by $2.4 million and, therefore, recorded the $2.4 million as deferred revenue that we are recognizing in the same manner as the $20.0 million upfront payment and research funding. We are eligible to receive certain contingent payments with respect to each target subject to the Immuno-Oncology Research Collaboration and royalties on sales of products related to such targets, if any. In December 2017, we recognized $5.0 million related to a developmental contingent payment. The Immuno-Oncology Research Collaboration will terminate upon the expiration of all payment obligations under the collaboration. In addition, BMS may terminate the Immuno-Oncology Research Collaboration in its entirety or on a collaboration target-by-collaboration target basis at any time with advance written notice, and either party may terminate the collaboration in its entirety or on a collaboration target-by-collaboration target basis with written notice for the other party’s material breach if such other party fails to timely cure the breach or immediately upon certain insolvency events. We identified one performance obligation under the Immuno-Oncology Research Collaboration for the research license to access our technology, the exclusive commercial license and research activities. BMS’s options to select additional collaboration targets are not priced at a discount and therefore do not represent performance obligations for which the transaction price would be allocated. The transaction price of $36.1 million includes the $20.0 million non-refundable upfront fee, $13.7 million of research funding and $2.4 million of equity premium. We concluded that the transaction price should not include the variable consideration related to maintenance fees and unachieved clinical and regulatory development milestones as this consideration was considered to be constrained as it is probable that the inclusion of such variable consideration could result in a significant reversal in revenue in the future. We will recognize any consideration related to sales-based payments (including milestones and royalties) when the related sales occur, as we have determined that these amounts relate predominantly to the license granted and therefore will be recognized on the later to occur of satisfaction of the performance obligation, or the occurrence of the related sales. We will re-evaluate the transaction price at each reporting period. For year ended December 31, 2018, no adjustments were made to the transaction price. Upon adoption of Topic 606, we recognized an additional $0.7 million of revenue, through a decrease to deferred revenue and an increase to beginning retained earnings, based on the difference between the input method currently used under Topic 606 and the ratable recognition method previously used under Topic 605. Under the input method, we recognize revenue on the basis of our efforts or inputs applicable to the satisfaction of a performance obligation (e.g., resources consumed, labor hours expended, costs incurred, or time elapsed) relative to the total expected inputs applicable to the satisfaction of that performance obligation. We concluded that we will recognize revenue based on actual costs incurred as a percentage of total budgeted costs as we complete our performance obligation. Revenue recognized from the performance obligation was $6.1 million for the year ended December 31, 2018. Through December 31, 2018, we have recognized $34.6 million of the transaction price as collaboration revenue under the agreement. We will recognize the remaining transaction price of $1.5 million as revenue under the input method over the estimated performance period. For the years ended December 31, 2018, 2017, and 2016, we recognized $6.1 million, $12.0 million and $7.7 million, respectively, of revenue under the Immuno-Oncology Research Collaboration. As of December 31, 2018 and 2017, we had deferred revenue relating to the immuno-oncology research collaboration of $1.5 million and $6.3 million, respectively. License and Collaboration Agreement On October 14, 2015, we entered into a license and collaboration agreement, or the Cabiralizumab Collaboration Agreement, pursuant to which we granted BMS exclusive global rights to develop and commercialize certain colony stimulating factor-1 receptor, or CSF1R, antibodies, including our monoclonal CSF1R inhibiting antibody that we refer to as cabiralizumab, and all modifications, derivatives, fragments, or variants of such antibodies, each of which we refer to as a licensed antibody. Under the terms of the Cabiralizumab Collaboration Agreement, BMS is responsible, at its expense, for developing products containing licensed antibodies, each of which we refer to as a licensed product, under a development plan, subject to our option, at our own expense, to conduct certain studies, including registration-enabling studies to support approval of cabiralizumab. BMS is responsible for manufacturing and commercializing each licensed product and we will retain rights to a U.S. co-promotion option. The supersedes the clinical trial collaboration agreement we entered into with BMS in November 2014, or the Original Collaboration Agreement. We assessed the two agreements separately as standalone agreements under Topic 606. We received an upfront payment of $30.0 million from BMS in December 2014 in connection with our entry into the Original Collaboration Agreement. We are completing our Phase 1a/1b clinical trial to evaluate the safety, tolerability and preliminary efficacy of combining Opdivo Under the Original Collaboration Agreement, we identified one performance obligation for the execution of a Phase 1a/1b clinical trial of cabiralizumab in combination with Opdivo We used the input method to measure progress toward completion of the performance obligation and concluded that we will recognize revenue based on actual costs incurred by our CRO, as a percentage of total budgeted costs as we complete our performance obligation. We will recognize revenue from reimbursements when we have the right to invoice BMS. No adjustment was necessary upon adoption of Topic 606. We recognized $6.6 million of the transaction price as revenue for the year ended December 31, 2018. Total revenue recognized for reimbursements for the year ended December 31, 2018, was $6.9 million. Through December 31, 2018, we recognized $24.8 million of the transaction price as collaboration revenue under the Original Collaboration Agreement. The remaining transaction price of $5.2 million is recorded in deferred revenue as of December 31, 2018 and will be recognized as revenue under the input method over the estimated performance period. Under the Cabiralizumab Collaboration Agreement The $350.0 million non-refundable upfront fee was fully recognized concurrent with the transfer of the license and know-how in 2015. As such, no adjustment to revenue was necessary under Topic 606. In January 2018, we recognized $25.0 million related to a milestone achieved for the dosing of the first patient in BMS’s randomized Phase 2 clinical trial of cabiralizumab in combination with Opdivo For the years ended December 31, 2018, 2017, and 2016, we recognized $38.4 million, $23.7 million and $14.4 million, respectively, of revenue under the license and collaboration agreements. As of December 31, 2018 and 2017, we had deferred revenue relating to the license and collaboration agreements of $5.2 million and $11.8 million, respectively. Zai Lab China License and Collaboration Agreement In December 2017, we entered into a license and collaboration agreement, or the China Collaboration Agreement, with Zai Lab, pursuant to which we granted Zai Lab an exclusive license to develop and commercialize bemarituzumab in China, Hong Kong, Macau and Taiwan. Under the terms of the China Collaboration Agreement non-refundable and non-creditable upfront fee ($4.2 million after netting of value-added tax withholdings of $0.8 million) in January 2018. Pursuant to the China collaboration agreement, with respect to each licensed product, we are eligible to receive up to $39.0 million of specified developmental and regulatory milestone payments. Zai Lab will also be obligated to pay us a royalty, on a licensed product-by-licensed product and region-by-region basis. In addition, Zai Lab agreed to reimburse us for certain global development activities, which is limited to a maximum of $10.0 million, and certain We identified the following performance obligations: (1) license grant to Zai Lab together with the transfer of licensed know-how, development drug supply and global development activities, or the License Grant and (2) development of companion diagnostics. Zai Lab has the option to purchase commercial drug supply from us pursuant to a separate commercial supply agreement to be negotiated in the future. The FIGHT trial We use the input method to measure progress toward completion of the performance obligation for the License Grant. We concluded that revenue will be recognized based on actual costs incurred by our CRO as a percentage of total budgeted costs as we complete our performance obligation. We will recognize revenue from reimbursements for the development of companion diagnostics when we have the right to invoice Zai Lab. No adjustment was necessary upon adoption of Topic 606. For the year ended December 31, 2018, revenue recognized for the License Grant was $1.7 million. Total revenue recognized for the companion diagnostics development performance obligation was $3.3 million. Of the remaining transaction price of $12.9 million, we recorded $5.2 million in deferred revenue, which we will recognize over the estimated performance period for satisfaction of the performance obligations. The remaining $8.7 million of the transaction price will be recorded in deferred revenue when invoiced as we complete global development activities. GlaxoSmithKline LLC Respiratory Diseases and Muscle Diseases Collaborations In April 2012, we entered into a research collaboration and license agreement, or the Respiratory Diseases Collaboration, with Glaxo Group Limited, or GSK, to identify new therapeutic approaches to treat refractory asthma and chronic obstructive pulmonary disease, or COPD, with a particular focus on identifying novel protein therapeutics and antibody targets. In January 2016, we amended our Respiratory Diseases Collaboration to extend the research term by three months to July 2016 to allow additional validation of the protein targets we discovered and to increase the research funding. In July 2010, we entered into a research collaboration and license agreement, or the Muscle Diseases Collaboration, with GlaxoSmithKline LLC, to identify potential drug targets and drug candidates to treat skeletal muscle diseases. We conducted three customized cell-based screens and one in vivo Based on our assessment of the Respiratory Diseases Collaboration and the Muscle Disease Collaboration under Topic 606, we identified one performance obligation under each collaboration for the research license and research activities. The non-refundable upfront fees, the equity premiums and the variable consideration for research activities are included as part of the transaction prices for each collaboration. The clinical and regulatory development milestone payments have not been included in the transaction prices, as all such milestone amounts are fully constrained. We will recognize any consideration related to sales-based payments (including milestones and royalties) when the related sales occur, as we have determined that these amounts relate predominantly to the license granted and therefore will be recognized on the later to occur of satisfaction of the performance obligation, or the occurrence of the related sales. Under the Respiratory Diseases Collaboration, additional research funding that GSK had the option to add was also not included in the transaction price. As the Muscle Diseases Collaboration with GlaxoSmithKline LLC terminated in April 2018, we are no longer eligible to receive milestone payments or royalties under that collaboration. We will re-evaluate the transaction price for the Respiratory Diseases Collaboration in each reporting period as uncertain events are resolved and other changes in circumstances occur. For year ended December 31, 2018, no adjustments were made to the transaction prices of the collaborations with GSK or GlaxoSmithKline LLC. Under the Respiratory Diseases Collaboration and the Muscle Diseases Collaboration, the non-refundable upfront fees, the equity premiums and the payment for research activities were fully recognized in 2016 and 2014, respectively. As the performance obligations were fully satisfied in prior years, no adjustment to revenue was necessary under Topic 606. For the years ended December 31, 2018, 2017 and 2016, we recognized $0, $0.5 million and $1.8 million of milestone revenue, respectively, and $0, $0 and $3.2 million of revenue for progress made toward the performance obligation Respiratory Diseases Collaboration. UCB Fibrosis and CNS Collaboration In March 2013, we entered into a research collaboration and license agreement, or the Fibrosis and CNS Collaboration, with UCB Pharma, S.A., or UCB, to identify potential biologics targets and therapeutics in the areas of fibrosis-related immunologic diseases and central nervous system, or CNS, disorders. Under the terms of the Fibrosis and CNS Collaboration Fibrosis and CNS Collaboration The agreement will terminate upon the expiration of the royalty terms of any products that incorporate or target a protein exclusively licensed under the collaboration. In addition, UCB may terminate this agreement at any time with advance written notice, and either party may terminate the agreement with written notice for the other party’s material breach if such other party fails to timely cure the breach or upon certain insolvency events. Based on our assessment of the Fibrosis and CNS Collaboration under Topic 606, we identified research activities as our only performance obligation. UCB’s options to select additional collaboration targets and to license exclusive rights to selected targets are not priced at a discount and therefore do not represent performance obligations for which the transaction price would be allocated. The transaction price of $15.6 million includes the $6.0 million non-refundable upfront fee, the $6.6 million technology access fee, the $1.0 million reimbursement for reagent costs and the $2.0 million of research funding. We have not included the clinical and regulatory development milestone payments in the transaction price as all such milestone amounts are fully constrained. We will recognize any consideration related to sales-based payments (including milestones and royalties) when the related sales occur, as we have determined that these amounts relate predominantly to the license granted and therefore will be recognized on the later to occur of satisfaction of the performance obligation, or the occurrence of the related sales. We will re-evaluate the transaction price in each reporting period as uncertain events are resolved and other changes in circumstances occur. For the year ended December 31, 2018, there was no change in the transaction price. Upon adoption of Topic 606, we recognized an additional $0.6 million of revenue through a decrease to deferred revenue and an increase to beginning retained earnings, based on the difference between the input method currently used under Topic 606 and the ratable recognition method previously used under Topic 605. We use the input method to measure progress toward completion of the performance obligation and concluded that revenue will be recognized based on actual full time equivalent labor hours expended as a percentage of total budgeted costs. The $0.6 million adjustment recorded upon the adoption of Topic 606 recognized the remainder of the transaction price. During 2018, 2017 and 2016, we recognized $0.3 million, $0.3 million and $0.4 million in target evaluation and selection fees, respectively. For the years ended December 31, 2018, 2017 and 2016, revenue recognized for the performance obligation was $0, $3.0 million and $3.1 million, respectively. As of December 31, 2017, we had deferred revenue of $0.6 million which was fully recognized upon adoption of Topic 606.

Acquired Technologies

Acquired Technologies	12 Months Ended
	Dec. 31, 2018
Research And Development [Abstract]
Acquired Technologies	9. Acquired Technologies Galaxy Biotech, LLC In December 2011, we entered into an exclusive license agreement with Galaxy Biotech, LLC, or Galaxy, for the development, manufacturing, and commercialization of certain anti-FGFR2b monoclonal antibodies. Under the terms of the agreement, we agreed to pay Galaxy an upfront license payment of $3.0 million. We paid the upfront payment in two equal installments in January 2012 and July 2012. As we had full access to the technology and materials upon execution of the agreement, the lead compound was in an early stage of development, and the underlying technology has no alternative future uses, we recorded the entire upfront payment to research and development expenses in our statement of operations for the year ended December 31, 2011. We are also required to make additional payments based upon the achievement of certain intellectual property, development, regulatory, and commercial milestones, as well as royalties on future net sales of products resulting from development of this purchased technology, if any. In May 2016, we amended the license agreement to revise certain milestone definitions, reduce certain milestone payments and add certain development-related milestone payments that were triggered by dosing of certain patients in the Phase 1 clinical trial of bemarituzumab. In May 2017, we further amended the license agreement to align the net sales definition under the agreement to the net sales definition under any sublicense we may grant under the agreement and to amend the termination provisions to allow for a direct license between Galaxy and any sublicensee upon termination of the agreement. BioWa, Inc. and Lonza Sales AG In February 2012, we entered into a license agreement with BioWa, Inc. and Lonza Sales AG, or BioWa-Lonza, pursuant to which BioWa-Lonza granted us a non-exclusive license to use their Potelligent ® In November 2015, we entered into a separate license agreement for the same technology with BioWa-Lonza for our FPA150 antibody program. We are obligated to pay BioWa-Lonza aggregate milestone payments of up to $24.5 million and $25.4 million, respectively, for development, regulatory and commercialization milestones achieved in our bemarituzumab and FPA150 antibody programs. We are also obligated to pay BioWa-Lonza tiered royalties on net sales up to mid-single digit percentages of the proceeds of such sales. We made milestone payments to BioWa-Lonza under both agreements totaling $1.2 million, $0 and $0, respectively, in 2018, 2017 and 2016. Our license agreements with BioWa-Lonza will remain in effect until the expiration of our royalty obligations under each agreement, unless earlier terminated. For each licensed product under each agreement, we are obligated to pay BioWa-Lonza royalties on net sales of such licensed product on a country-by-country basis for the longer of the life of the licensed patents covering such licensed product in such country or 10 years after the first commercial sale of such licensed product in a major market country, which includes the United States . INBRX 110 LP In July 2015, we entered into a research collaboration and license agreement with INBRX 110 LP, or Inhibrx, to obtain (a) an exclusive, worldwide license to antibodies to GITR for therapeutic and diagnostic uses, and (b) an exclusive option to obtain exclusive, worldwide licenses to multi-specific antibodies developed by Inhibrx that bind to both GITR and other targets. Pursuant to the agreement, we paid Inhibrx an upfront fee of $10.0 million for the license and for services provided by Inhibrx related to a research cell bank in July 2015. We recorded an expense of $5.0 million for a milestone payment to Inhibrx when the milestone was achieved in May 2017. We expense payments for the acquisition and development of technology as research and development cost if, at the time of payment, the technology is under development, is not approved by the FDA or other regulatory agencies for marketing, has not reached technical feasibility, or otherwise has no foreseeable alternative future use. In accordance with this policy, we expensed the $8.0 million that we determined to be related to the license upon our entry into the agreement in July 2015 as research and development expense. In accordance with the ASC 730, Research and Development Costs On August 28, 2017, we delivered to Inhibrx written notice of termination of the agreement for convenience. Pursuant to the terms of the agreement, the termination became effective on December 27, 2017.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	10. Income Taxes For the year ended December 31, 2018, we did not record any income tax expense as compared to an income tax expense of $1.7 million for the year ended December 31, 2017 and an income tax benefit of $31.0 million for the year ended December 31, 2016. For the year ended December 31, 2017, the income tax expense related to deficiency interest was based on the Internal Revenue Service reducing our tentative net operating loss carryback refund claim filed in March 2017. For the year ended December 31, 2016, the federal tax benefit represents the reversal of the federal tax provided in 2015 due to our ability to carryback federal tax attributes generated in 2016, but not in an amount that was lower than any minimum taxes as provided under federal law. The components of our income tax expense (benefit) were as follows: Year Ended December 31, 2018 2017 2016 Current tax expense (benefit) Federal $ — $ 1,703 $ (40,740 ) State — 1 (5,340 ) Total current expense (benefit) — 1,704 (46,080 ) Deferred tax expense Federal — — 15,032 State — — — Total deferred tax expense — — 15,032 Total tax expense (benefit) $ — $ 1,704 $ (31,048 ) The income tax expense (benefit) differs from the amount computed by applying the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2018 2017 2016 Federal statutory income tax $ (29,495 ) $ (51,981 ) $ (33,862 ) State statutory income tax 1 1 (3,471 ) Stock compensation 3,698 (4,847 ) 715 Nontaxable equity premiums (85 ) (168 ) (248 ) Change in valuation allowance 38,953 41,633 12,152 Remeasurement of deferred taxes — 27,122 — Research and orphan drug credits (13,192 ) (11,029 ) (8,029 ) Interest charge, net of federal benefit — 1,107 — Other permanent items 120 (134 ) 1,695 Income tax expense (benefit) $ — $ 1,704 $ (31,048 ) On December 22, 2017, the Tax Cuts and Jobs Act of 2017, or the Tax Act, was signed into law. The Tax Act reduces the corporate tax rate from a top marginal rate of 35% to a flat rate of 21%. Although the Tax Act is generally effective on January 1, 2018, GAAP requires recognition of the tax effects of new legislation during the reporting period that includes the enactment date, which was December 22, 2017. Because of the impacts of the Tax Act, the SEC issued Staff Accounting Bulletin No. 118 Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB 118) that allows us to record provisional amounts for those impacts, with the requirement that the accounting be completed in a period not to exceed one year from the date of enactment. As a result, as of December 31, 2017, we performed a provisional estimate of the effect of the Tax Act in the financial statements. In the fourth quarter of 2018, we completed our analysis to determine the effect of the Tax Act. No material adjustments were noted from the completion of the analysis as of December 31, 2018. The primary impact of the Tax Act resulted from the re-measurement of deferred tax assets and liabilities due to the change in the corporate tax rate, reducing our deferred tax assets by $27.1 million with a corresponding reduction in our valuation allowance, which had no effect on our effective tax rate. The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets consist of the following (in thousands): As of December 31, 2018 2017 Net operating loss carryforwards $ 60,566 $ 39,405 Research and orphan drug credits 58,614 42,070 Deferred revenue 2,410 3,701 Stock-based compensation 7,698 7,017 Capitalized license and depreciation basis differences 3,412 892 Reserves, accruals and tenant improvement allowances 5,639 5,738 Total deferred tax assets 138,339 98,823 Less: valuation allowance (134,492 ) (94,315 ) Net deferred tax assets $ 3,847 $ 4,508 Capitalized license and depreciation basis differences (3,396 ) (4,069 ) Prepaid expenses (451 ) (439 ) Total deferred tax liabilities $ (3,847 ) $ (4,508 ) Total net deferred tax assets $ — $ — Based on all available objective evidence, we determined it is more likely than not that we will not fully realize all our net deferred tax assets. The available objective evidence considered was our inability to further recover any taxes previously paid and expectation of future taxable income. Accordingly, we recorded a valuation allowance against all our net deferred tax assets for the years ended December 31, 2018 and 2017. We will continue to maintain a full valuation allowance on our net deferred tax assets until there is sufficient positive evidence to support the reversal of all or some portion of this allowance. Our valuation allowance increased by $40.2 million and $47.4 million, respectively, during 2018 and 2017. At December 31, 2018, we had approximately $257.0 million of federal net operating losses available for future use that expire beginning in 2024 and federal research and Orphan Drug credits of approximately $52.2 million available for future use that expire beginning in 2026. At December 31, 2018, we also had approximately $155.1 million of state net operating losses available for future use that expire beginning in 2028 and state research credits of approximately $21.4 million that have no expiration date. Utilization of net operating loss and tax credit carryforwards may be subject to an annual limitation due to ownership change limitations provided by the Internal Revenue Code and similar state provisions. Annual limitations may result in expiration of net operating loss and tax credit carryforwards before some or all of such amounts have been utilized. We had $16.7 million, $13.6 million and $9.4 million of unrecognized tax benefits as of December 31, 2018, 2017 and 2016, respectively. The unrecognized tax benefits are primarily tax credits for all years and state net operating loss carryover related for certain prior years. As of December 31, 2018, we recorded no interest or penalties related to income taxes. Comparatively, we recorded $1.7 million of interest as of December 31, 2017. A reconciliation of our unrecognized tax benefits for the years ended December 31, 2018, 2017 and 2016 Unrecognized Income Tax Benefits Balance as of December 31, 2015 $ 3,432 Additions for prior year tax positions 4,394 Additions for current year tax positions 1,577 Balance as of December 31, 2016 9,403 Additions for prior year tax positions 691 Additions for current year tax positions 3,490 Balance as of December 31, 2017 13,584 Additions for prior year tax positions 622 Additions for current year tax positions 2,454 Balance as of December 31, 2018 $ 16,660 In the event we are able to recognize these uncertain positions, most of the $16.7 million of the unrecognized tax benefits would reduce our effective tax rate. We currently have a full valuation allowance against our deferred tax assets, which would impact the timing of the effective tax rate benefit, should any of these uncertain positions be favorably settled in the future. We do not believe it is reasonably possible that our unrecognized tax benefits will significantly change within the next twelve months. We file U.S. and state income tax returns with varying statutes of limitations. The tax years from 2003 forward remain open to examination due to the carryover of unused net operating losses and tax credits. We have no ongoing tax examinations by tax authorities at this time.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	11. Commitments and Contingencies Operating Leases We entered into a lease agreement for our new corporate office and laboratory facility in December 2016, which we refer to as the lease. We moved into our new corporate office and laboratory facility in December 2017. The lease has an initial term of 10 years, beginning on the rent commencement date, with an option to extend the lease for an additional period of five years. We did not have to pay rent until the rent commencement date of January 1, 2018 and rent was reduced by 50% for the first six months. The lease contains scheduled rent increases over the lease term. We recognize the related rent expense for the lease on a straight-line basis over the term of the lease with the difference between the rent paid and the straight-line rent expense recorded as deferred rent. We received lease incentives totaling $14.4 million recorded as deferred rent from our landlord for a portion of the costs of leasehold improvements we made to the premises. We amortize the incentives on a straight-line basis over the term of the lease as a reduction of rent expense. As of December 31, 2018 and 2017, the unamortized leasehold improvement incentive totaled $12.2 million and $13.6 million, respectively. In addition, the lease required us to deliver an irrevocable standby letter of credit in an amount of $1.5 million to the landlord for the period commencing on the effective date of the agreement until at least 60 days after the expiration of the lease, subject to 50% reduction on January 1, 2023 if certain conditions are met. In July 2018, we entered into a lease agreement for the installation, operational qualifications and performance qualifications of four sequencing instruments to support our bemarituzumab program. The agreement has two 3-year terms based on delivery dates for the first three instruments in July 2018 and the fourth instrument in January 2019. The lease contains consistent rent payments over the terms of the lease. We recognize the related rent expense for the delivered instruments on a straight-line basis over the term of the lease. Rent expense for the years ended December 31, 2018, 2017 and 2016 was $5.9 million, $6.9 million, and $ million, respectively. Year ending December 31: 2019 7,315 2020 7,564 2021 7,705 2022 7,787 2023 8,064 2024 and on 35,166 Total estimated minimum payments $ 73,601 Indemnifications As permitted under Delaware law and in accordance with our bylaws, we have agreed to indemnify our officers and directors for certain events or occurrences, subject to certain limits, while the officer or director is or was serving at our request in such capacity. The term of the indemnification period is equal to the officer’s or director’s lifetime. The maximum amount of potential future indemnification is unlimited; however, we currently hold director and officer liability insurance. This insurance limits our exposure and may enable us to recover a portion of any future amounts paid. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period presented. We have certain agreements with service providers and other parties with which we do business that contain indemnification provisions pursuant to which we have agreed to indemnify the party against certain types of third-party claims. We accrue for known indemnification issues when a loss is probable and can be reasonably estimated. We would also accrue for estimated incurred but unidentified indemnification issues based on historical activity. As we have not incurred any indemnification losses to date, there were no accruals for or expenses related to indemnification issues for any period presented.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	12. Subsequent Events In January 2019, we implemented a corporate restructuring, or the restructuring, to focus our resources on our clinical development and late-stage research programs. Pursuant to the restructuring, we eliminated 41 employee positions, representing approximately 20% of our then-current headcount, primarily in areas relating to research, pathology and manufacturing We estimate approximately $2.0 million of pre-tax charges for severance and other costs related to the restructuring, primarily during the first quarter of 2019.

Selected Quarterly Financial In

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information	13. Selected Quarterly Financial Information (Unaudited) The following amounts are in thousands, except per share amounts: Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2018 2018 2018 2018 (Unaudited) Revenue $ 32,486 $ 7,580 $ 5,771 $ 4,031 Net loss (20,390 ) (34,060 ) (47,244 ) (38,753 ) Basic and diluted net loss per share (0.63 ) (0.99 ) (1.37 ) (1.12 ) Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2017 2017 2017 2017 (Unaudited) Revenue $ 10,135 $ 7,822 $ 8,333 $ 13,218 Net loss (33,443 ) (44,286 ) (43,282 ) (29,211 ) Basic and diluted net loss per share (1.21 ) (1.58 ) (1.54 ) (1.04 ) Basic and diluted net loss per share is computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share amounts may not equal annual basic and diluted net loss per share amounts.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes as of the date of the financial statements. The most significant estimates in the Company’s financial statements include the recognition of revenue, stock-based compensation, completeness of clinical trial accruals and income taxes. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid investments purchased with original maturities of three months or less at the date of purchase to be cash equivalents. Cash equivalents are recorded at face value, or cost, which approximates fair value.
Restricted Cash	Restricted Cash Restricted cash consists of a certificate of deposit held by our bank as collateral for a standby letter of credit in the same notional amount by our landlord to secure our obligations under our corporate office and laboratory facility lease entered in December 2016. We are required to maintain this restricted cash balance, the amount of which is subject to reduction starting on January 1, 2023, if certain conditions are met, for the duration of this lease
Marketable Securities	Marketable Securities All marketable securities have been classified as “available-for-sale” and are carried at fair value, based upon quoted market prices. We consider our available-for-sale portfolio as available for use in current operations. Accordingly, we classify certain investments as short-term marketable securities, even though the stated maturity date may be one year or more beyond the current balance sheet date. Unrealized gains and losses, net of any related tax effects, are excluded from earnings and are included in other comprehensive income or loss and reported as a separate component of stockholders’ equity or deficit until realized. Realized gains and losses and declines in value judged to be other than temporary, if any, on available-for-sale securities are included in other income (expense), net. The cost of securities sold is based on the specific identification method. We adjust the amortized cost of securities for amortization of premiums and accretion of discounts to maturity. We include interest on short-term investments in interest income. In accordance with our investment policy, management invests to diversify credit risk and only invests in debt securities with high credit quality, including U.S. government securities. We periodically evaluate whether declines in the fair value of our investments below their cost are other than temporary. The evaluation includes consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities, and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. If we determine that the decline in fair value of an investment is below its accounting basis and this decline is other than temporary, we would reduce the carrying value of the security we hold and record a loss for the amount of such decline. We have not recorded any realized losses or declines in value judged to be other than temporary on our investments in debt securities.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject us to significant concentrations of credit risk consist primarily of cash and cash equivalents and marketable securities. Cash and cash equivalents and marketable securities are invested through banks and other financial institutions in the United States. Such deposits in the United States may be in excess of insured limits.
Fair Value of Financial Instruments	Fair Value of Financial Instruments We determine the fair value of financial and nonfinancial assets and liabilities using the fair value hierarchy, which describes three levels of inputs that may be used to measure fair value, as follows: Level 1 —Quoted prices in active markets for identical assets or liabilities; Level 2 —Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities, quoted prices for identical or similar assets or liabilities 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. For our marketable securities, we review trading activity and pricing as of the measurement date. When sufficient quoted pricing for identical securities is not available, we use market pricing and other observable market inputs for similar securities obtained from various third-party data providers. These inputs either represent quoted prices for similar assets in active markets or have been derived from observable market data; and Level 3 —Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. We determine the fair value of Level 1 assets using quoted prices in active markets for identical assets. We review trading activity and pricing for Level 2 investments as of each measurement date. Level 2 inputs, obtained from various third-party data providers, represent quoted prices for similar assets in active markets and were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data. There were no transfers between Level 1 and Level 2 securities in the periods presented. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3 within the valuation hierarchy. We do not have any assets or liabilities measured using Level 3 inputs as of December 31, 2018. The following table summarizes our financial instruments that were measured at fair value on a recurring basis by level of input within the fair value hierarchy defined above (in thousands): December 31, 2018 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 40,849 $ 40,849 $ — $ — U.S. Treasury securities 104,140 104,140 — — Agency bonds 53,999 53,999 — — Corporate bonds 11,893 — 11,893 — Commercial paper 56,152 — 56,152 — Certificate of deposit 1,543 — 1,543 — Total $ 268,576 $ 198,988 $ 69,588 $ — December 31, 2017 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 31,802 $ 31,802 $ — $ — U.S. Treasury securities 232,900 232,900 — — Certificate of deposit 1,543 — 1,543 — Total $ 266,245 $ 264,702 $ 1,543 $ —
Property and Equipment	Property and Equipment Property and equipment are stated at cost and depreciated using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of their estimated useful lives or the related lease term.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets include property and equipment. We review the carrying value of long-lived assets for impairment whenever events or changes in circumstances indicate that the assets may not be recoverable. We recognize an impairment loss when the total estimated future cash flows expected to result from the use of the asset and its eventual disposition are less than the carrying amount. Through December 31, 2018, there have been no such impairment losses.
Revenue Recognition	Revenue Recognition Effective January 1, 2018, we adopted Financial Accounting Standards Board, or FASB, Accounting Standard Update, or ASU , Revenue from Contracts with Customers (Topic 606) The terms of our collaborative research and development agreements include upfront and license fees, research, development and other funding or reimbursements, milestone and other contingent payments for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of commercialized products. Arrangements that include upfront payments may require deferral of revenue recognition to a future period until we perform obligations under these arrangements. We record research and development funding payable to us as accounts receivable when our right to consideration is unconditional. The event-based milestone and other contingent payments represent variable consideration, and we use the most likely amount method to estimate this variable consideration. Given the high degree of uncertainty around occurrence of these events, we determine the milestone and other contingent amounts to be fully constrained until the uncertainty associated with these payments is resolved. We will recognize revenue from sales-based royalty payments when or as the sales occur. We will re-evaluate the transaction price in each reporting period as uncertain events are resolved and other changes in circumstances occur. A performance obligation is a promise in a contract to transfer a distinct good or service and is the unit of accounting in Topic 606. A contract’s transaction price is allocated among each distinct performance obligation based on relative standalone selling price and recognized as revenue when, or as, the applicable performance obligation is satisfied. Under Topic 606, we elected to use the practical expedient permitted related to adoption, which does not require us to disclose certain information regarding our remaining performance obligations as of the end of the reporting period prior to the initial date of adoption. Additionally, we elected the practical expedient for certain research and development funding which allows us to recognize revenue in the amount for which we have a right to invoice if our right to consideration is an amount that corresponds directly to the value of our performance completed to date. As a result, we effectively bypass the steps of determining the transaction price and allocating that transaction price to the performance obligation.
Research and Development Expenses	Research and Development Expenses Research and development expenses consist of costs we incur for our own and for sponsored and collaborative research and development activities. Research and development costs are expensed as incurred. Research and development costs consist of salaries and benefits, including associated stock-based compensation, laboratory supplies and facility costs, as well as fees paid to other entities that conduct certain research and development activities on our behalf. We estimate preclinical study and clinical trial expenses based on the services performed pursuant to contracts with research institutions and contract research organizations, or CROs, and clinical manufacturing organizations, or CMOs, that conduct and manage preclinical studies and clinical trials on our behalf based on actual time and expenses incurred by them. Further, we accrue expenses related to clinical trials based on the level of patient activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. If we do not identify costs that we have begun to incur or if we underestimate or overestimate the level of services performed or the costs of these services, our actual expenses could differ from our estimates. We expense payments for the acquisition and development of technology as research and development costs if, at the time of payment, the technology: is under development; is not approved by the U.S. Food and Drug Administration or other regulatory agencies for marketing; has not reached technical feasibility; or otherwise has no foreseeable alternative future use.
Stock-Based Compensation	Stock-Based Compensation We recognize compensation expense using a fair-value-based method for costs related to all share-based payments, including restricted stock awards, or RSAs, and stock option awards. For RSAs, stock-based compensation cost is based on the closing market value of our common stock at the date of grant and is recognized as expense ratably over the requisite service period. For stock option awards, stock-based compensation cost is measured at the grant date, based on the fair-value-based measurement of the award estimated using the Black-Scholes option-pricing model, and is recognized as expense over the requisite service period on a straight-line basis. We account for forfeitures as they occur by reversing any expense recognized for unvested awards.
Income Taxes	Income Taxes We account for income taxes using the liability method, under which deferred tax assets and liabilities are determined based on differences between financial reporting and tax basis of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Valuation allowances are provided when the expected realization of the deferred tax assets does not meet the more-likely-than-not criteria. As a result, deferred tax assets at the end of 2018 and 2017 are subject to a full valuation allowance. We are required to determine whether it is more likely than not that a tax position will be sustained upon examination by the appropriate taxing authorities before any part of the benefit can be recorded in the financial statements. It is our practice to recognize interest and penalties related to unrecognized tax benefits, if any, as a component of income tax expense.
Accounting Pronouncements Adopted in 2018	Accounting Pronouncements Adopted in 2018 In May 2014, FASB issued Topic 606, which supersedes nearly all existing revenue recognition guidance under U.S. generally accepted accounting principles , or GAAP. FASB subsequently issued amendments to Topic 606 that have the same effective date and transition date. The core principle of Topic 606 is to recognize revenues when promised goods or services are transferred to customers in an amount that reflects the consideration that is expected to be received for those goods or services. Topic 606 defines a five-step process to achieve this core principle and, in as a result, more judgment and estimates may be required in the course of the revenue recognition process, including with respect to identifying performance obligations in a contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. We adopted Topic 606, effective January 1, 2018, using the modified retrospective transition method, in which the new standard is applied as of the date of initial adoption. We applied the standard to contracts that were not completed at the date of initial application. We recorded the cumulative effect of initially applying the standard as an adjustment to the opening balance of retained earnings. The adoption of the new revenue recognition guidance resulted in a decrease of $1.4 million to deferred revenue and an increase of $1.4 million to retained earnings as of January 1, 2018. Additionally, we determined that the classification between deferred revenue, current portion, and deferred revenue, long-term portion, changed as a result of adoption of Topic 606. We concluded that we will classify deferred revenue for all licensing and collaboration arrangements as deferred revenue, long-term portion, and will reclassify to deferred revenue, current portion, when the remaining term of the estimated performance period is one year or less. Our adoption of Topic 606 effective January 1, 2018 affected the following financial statement line items: Condensed Statements of Operations Year Ended December 31, 2018 (in thousands, except per share data) Under Topic 606 Under Topic 605 Effect of change Collaboration and license revenue $ 49,868 $ 52,329 $ (2,461 ) Operating expenses 196,023 196,023 — Operating loss $ (146,155 ) $ (143,694 ) $ (2,461 ) Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Net loss per share applicable to common stockholders - basic and diluted $ (4.13 ) $ (4.06 ) $ (0.07 ) Condensed Balance Sheets December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Receivables from collaborative partner $ 5,096 $ 5,096 $ — Deferred revenue, current portion 1,428 8,187 (6,759 ) Deferred revenue, long-term portion 10,465 2,618 7,847 Accumulated deficit (294,681 ) (293,593 ) (1,088 ) Condensed Statement of Cash Flows Year Ended December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Decrease in deferred revenue in connection with Topic 606 adoption 1,373 — 1,373 Changes in operating assets and liabilities Receivables from collaborative partner 8,037 8,037 — Deferred revenue (11,043 ) (12,131 ) 1,088 Cash, cash equivalents and restricted cash at beginning of period 61,333 61,333 — Cash, cash equivalents and restricted cash at end of period 45,496 45,496 — In May 2017, FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718) – Scope of Modification Accounting In November 2016, FASB issued ASU 2016-18, Statement of Cash Flows (Topic 230) – Restricted Cash In June 2018, FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718) – Improvements to Nonemployee Share-Based Payment Accounting Subtopic 505-50, Equity–Equity-Based Payments to Non-Employees
Accounting Pronouncements Not Yet Adopted	Accounting Pronouncements Not Yet Adopted In November 2018, FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808) clarifies when certain transactions between collaborative arrangement participants should be accounted for under Topic 606 and incorporates unit-of-account guidance consistent with Topic 606 to aid in this determination. ASU 2018-18 will become effective January 1, 2020 and will apply to all annual and interim reporting periods thereafter. Early adoption is permitted. ASU 2018-18 should generally be applied retrospectively to the date of initial application of Topic 606. We do not anticipate that the adoption of this standard will have a material effect on our financial statements In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. These amendments eliminate, modify, or integrate into other SEC requirements certain disclosure rules. Among the amendments is the requirement to present an analysis of changes in stockholders’ equity in the interim financial statements included in quarterly reports on Form 10-Q. The analysis, which can be presented as a footnote or separate statement, is required for the current and comparative quarter and year-to-date interim periods. The amendments are effective for all filings made on or after November 5, 2018. In light of the anticipated timing of effectiveness of the amendments and expected proximity of effectiveness to the filing date for most filers’ quarterly reports, the SEC’s Division of Corporate Finance issued a Compliance and Disclosure Interpretation related to Exchange Act Forms, or CDI – Question 105.09, that provides transition guidance related to this disclosure requirement. CDI – Question 105.09 states that the SEC would not object if the filer’s first presentation of the changes in shareholders’ equity is included in its quarterly report on Form 10-Q for the quarter that begins after the effective date of the amendments. As such, we adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity beginning the first quarter of 2019. We do not anticipate that the adoption of these SEC amendments will have a material effect on our financial position, results of operations, cash flows or shareholders’ equity. In August 2018, FASB issued ASU 2018-13, Fair Value Measurement - Disclosure Framework (Topic 820) In June 2016, FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326)

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Summary of Financial Instruments Measured at Fair Value	The following table summarizes our financial instruments that were measured at fair value on a recurring basis by level of input within the fair value hierarchy defined above (in thousands): December 31, 2018 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 40,849 $ 40,849 $ — $ — U.S. Treasury securities 104,140 104,140 — — Agency bonds 53,999 53,999 — — Corporate bonds 11,893 — 11,893 — Commercial paper 56,152 — 56,152 — Certificate of deposit 1,543 — 1,543 — Total $ 268,576 $ 198,988 $ 69,588 $ — December 31, 2017 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 31,802 $ 31,802 $ — $ — U.S. Treasury securities 232,900 232,900 — — Certificate of deposit 1,543 — 1,543 — Total $ 266,245 $ 264,702 $ 1,543 $ —
ASU 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Summary of Guidance Effect under Topic 605 before Adoption of Topic 606 on Financial Statement	Our adoption of Topic 606 effective January 1, 2018 affected the following financial statement line items: Condensed Statements of Operations Year Ended December 31, 2018 (in thousands, except per share data) Under Topic 606 Under Topic 605 Effect of change Collaboration and license revenue $ 49,868 $ 52,329 $ (2,461 ) Operating expenses 196,023 196,023 — Operating loss $ (146,155 ) $ (143,694 ) $ (2,461 ) Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Net loss per share applicable to common stockholders - basic and diluted $ (4.13 ) $ (4.06 ) $ (0.07 ) Condensed Balance Sheets December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Receivables from collaborative partner $ 5,096 $ 5,096 $ — Deferred revenue, current portion 1,428 8,187 (6,759 ) Deferred revenue, long-term portion 10,465 2,618 7,847 Accumulated deficit (294,681 ) (293,593 ) (1,088 ) Condensed Statement of Cash Flows Year Ended December 31, 2018 (in thousands) Under Topic 606 Under Topic 605 Effect of change Net loss $ (140,447 ) $ (137,986 ) $ (2,461 ) Decrease in deferred revenue in connection with Topic 606 adoption 1,373 — 1,373 Changes in operating assets and liabilities Receivables from collaborative partner 8,037 8,037 — Deferred revenue (11,043 ) (12,131 ) 1,088 Cash, cash equivalents and restricted cash at beginning of period 61,333 61,333 — Cash, cash equivalents and restricted cash at end of period 45,496 45,496 —

Cash Equivalents and Marketab_2

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Cash And Cash Equivalents [Abstract]
Summary of Cash Equivalents and Marketable Securities	The following is a summary of our cash equivalents and marketable securities at December 31, 2018 and 2017 (in thousands): December 31, 2018 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 40,849 $ — $ — $ 40,849 U.S. Treasury securities 104,218 — (78 ) 104,140 Agency bonds 54,005 9 (15 ) 53,999 Corporate bonds 11,897 — (4 ) 11,893 Commercial paper 56,171 0 (19 ) 56,152 Total cash equivalents and marketable securities 267,140 9 (115 ) 267,034 Less: cash equivalents (40,849 ) — — (40,849 ) Total marketable securities $ 226,291 $ 9 $ (115 ) $ 226,185 December 31, 2017 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 31,802 $ — $ — $ 31,802 U.S. Treasury securities 233,376 — (476 ) 232,900 Total cash equivalents and marketable securities 265,178 — (476 ) 264,702 Less: cash equivalents (31,802 ) — — (31,802 ) Total marketable securities $ 233,376 $ — $ (476 ) $ 232,900
Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity	As of December 31, 2018, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands): Estimated Amortized Fair Cost Value Debt securities maturing: In one year or less $ 226,291 $ 226,185 Total marketable securities $ 226,291 $ 226,185

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2018 2017 Computer equipment and software $ 2,403 $ 1,892 Furniture and fixtures 968 947 Laboratory equipment 19,579 17,429 Leasehold improvements 22,175 22,175 $ 45,125 $ 42,443 Less: accumulated depreciation and amortization (16,407 ) (11,681 ) Property and equipment, net $ 28,718 $ 30,762

Other Accrued Liabilities (Tabl

Other Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables And Accruals [Abstract]
Schedule of Other Accrued Liabilities	Other accrued liabilities consist of the following (in thousands): December 31, 2018 2017 Clinical development $ 10,513 $ 12,580 Manufacturing 1,104 2,835 Trade payable 3,381 3,995 Unpaid leasehold improvements — 7,742 Other 350 367 Total accrued liabilities $ 15,348 $ 27,519

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Option Activity under Stock Plans and Related Information	The following table summarizes option activity under our stock plans and related information: Options Outstanding Weighted- Weighted- Average Average Exercise Remaining Aggregate Number Price Contractual Intrinsic of Shares Per Share Terms Value (in years) (in thousands) Balance at January 1, 2018 3,867,645 $ 30.35 Options granted 858,100 17.05 Options exercised (328,585 ) 8.18 Options forfeited (303,640 ) 34.20 Options expired (383,339 ) 35.69 Balance at December 31, 2018 3,710,181 28.37 7.05 $ 740,587 Options exercisable at December 31, 2018 2,195,470 26.95 6.03 740,587
Schedule of Restricted Stock Award Activity under Stock Plans and Related Information	The following table summarizes the RSA activity under our stock plans and related information: RSAs Outstanding Weighted-Average Number Grant-Date of Shares Fair Value Unvested balance at January 1, 2018 803,417 $ 40.24 RSAs granted 715,775 17.62 RSAs vested (392,136 ) 34.89 RSAs forfeited (247,026 ) 32.66 Unvested balance at December 31, 2018 880,030 26.36
Schedule of Stock-Based Compensation Expenses Recognized	Total stock-based compensation expense recognized was as follows: Year Ended December 31, (in thousands) 2018 2017 2016 Research and development $ 15,426 $ 18,285 $ 17,960 General and administrative 14,042 15,888 14,925 Total $ 29,468 $ 34,173 $ 32,885
Schedule of Stock Option Weighted-Average Assumptions	We estimated the fair value of each award using the Black-Scholes option-pricing model based on the date of grant of such award with the following assumptions: Options ESPP Year Ended December 31, Year Ended December 31, 2018 2017 2016 2018 2017 2016 Expected term (years) 5.5-6.3 5.5-6.3 5.5-6.3 0.5 0.5 0.5 Expected volatility 68-70% 66-70% 69-74% 47-94% 42-94% 47-57% Risk-free interest rate 2.6-2.9% 1.9-2.2% 1.3-1.8% 1.4-2.5% 1.0-1.4% 0.4-0.6% Expected dividend yield 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%

Earnings per Share (Tables)

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss	The following table sets forth the computation of basic and diluted net loss (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (140,447 ) $ (150,222 ) $ (65,697 ) Denominator: Denominator for basic loss per share - weighted-average shares 33,976 27,945 26,955 Denominator for diluted loss per share 33,976 27,945 26,955 Basic and diluted net loss per share $ (4.13 ) $ (5.38 ) $ (2.44 )
Securities Excluded from Calculation of Diluted Net Loss Per Share	We excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands): Year Ended December 31, 2018 2017 2016 Options to purchase common stock 3,710 3,843 2,981 RSAs 880 886 1,278 Total 4,590 4,729 4,259

License and Collaboration Agr_2

License and Collaboration Agreements (Tables)	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Summary of Changes in Balances of Contract Assets and Contract Liabilities	The following table presents changes during the year ended December 31, 2018 in the balances of our contract assets, including receivables from collaboration partners, and contract liabilities, including deferred revenue. (in thousands) Contract Assets Balance at January 1, 2018 $ 13,133 Additions 39,262 Deductions (47,299 ) Balance at December 31, 2018 $ 5,096 (in thousands) Contract Liabilities Balance at January 1, 2018 $ 21,563 Additions for advance billings 4,727 Deductions for performance obligations satisfied in current period (12,218 ) Deductions for performance obligations satisfied in the prior periods in connection with updates to the measure of progress (2,179 ) Balance at December 31, 2018 $ 11,893

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense (Benefit)	The components of our income tax expense (benefit) were as follows: Year Ended December 31, 2018 2017 2016 Current tax expense (benefit) Federal $ — $ 1,703 $ (40,740 ) State — 1 (5,340 ) Total current expense (benefit) — 1,704 (46,080 ) Deferred tax expense Federal — — 15,032 State — — — Total deferred tax expense — — 15,032 Total tax expense (benefit) $ — $ 1,704 $ (31,048 )
Schedule of Income Tax Expense (Benefit) Differs from Amount Computed by Applying Statutory Federal Income Tax Rate	The income tax expense (benefit) differs from the amount computed by applying the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2018 2017 2016 Federal statutory income tax $ (29,495 ) $ (51,981 ) $ (33,862 ) State statutory income tax 1 1 (3,471 ) Stock compensation 3,698 (4,847 ) 715 Nontaxable equity premiums (85 ) (168 ) (248 ) Change in valuation allowance 38,953 41,633 12,152 Remeasurement of deferred taxes — 27,122 — Research and orphan drug credits (13,192 ) (11,029 ) (8,029 ) Interest charge, net of federal benefit — 1,107 — Other permanent items 120 (134 ) 1,695 Income tax expense (benefit) $ — $ 1,704 $ (31,048 )
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets consist of the following (in thousands): As of December 31, 2018 2017 Net operating loss carryforwards $ 60,566 $ 39,405 Research and orphan drug credits 58,614 42,070 Deferred revenue 2,410 3,701 Stock-based compensation 7,698 7,017 Capitalized license and depreciation basis differences 3,412 892 Reserves, accruals and tenant improvement allowances 5,639 5,738 Total deferred tax assets 138,339 98,823 Less: valuation allowance (134,492 ) (94,315 ) Net deferred tax assets $ 3,847 $ 4,508 Capitalized license and depreciation basis differences (3,396 ) (4,069 ) Prepaid expenses (451 ) (439 ) Total deferred tax liabilities $ (3,847 ) $ (4,508 ) Total net deferred tax assets $ — $ —
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of our unrecognized tax benefits for the years ended December 31, 2018, 2017 and 2016 Unrecognized Income Tax Benefits Balance as of December 31, 2015 $ 3,432 Additions for prior year tax positions 4,394 Additions for current year tax positions 1,577 Balance as of December 31, 2016 9,403 Additions for prior year tax positions 691 Additions for current year tax positions 3,490 Balance as of December 31, 2017 13,584 Additions for prior year tax positions 622 Additions for current year tax positions 2,454 Balance as of December 31, 2018 $ 16,660

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Estimated Future Minimum Commitments for Non-cancelable Operating Leases	The estimated future minimum commitments under our non-cancelable operating leases are as follows (in thousands): Year ending December 31: 2019 7,315 2020 7,564 2021 7,705 2022 7,787 2023 8,064 2024 and on 35,166 Total estimated minimum payments $ 73,601

Selected Quarterly Financial _2

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Information	The following amounts are in thousands, except per share amounts: Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2018 2018 2018 2018 (Unaudited) Revenue $ 32,486 $ 7,580 $ 5,771 $ 4,031 Net loss (20,390 ) (34,060 ) (47,244 ) (38,753 ) Basic and diluted net loss per share (0.63 ) (0.99 ) (1.37 ) (1.12 ) Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2017 2017 2017 2017 (Unaudited) Revenue $ 10,135 $ 7,822 $ 8,333 $ 13,218 Net loss (33,443 ) (44,286 ) (43,282 ) (29,211 ) Basic and diluted net loss per share (1.21 ) (1.58 ) (1.54 ) (1.04 )

Business - Additional Informati

Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018Segment
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segment	1

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Fair value, assets, level 1 to level 2 transfers		$ 0
Fair value, assets, level 2 to level 1 transfer		0
Impairment losses on long lived assets		0
Decrease to deferred revenue		1,373,000
Increase to retained earnings		(294,681,000)	$ (155,607,000)
Deferred tax asset derecognized		4,200,000
ASU 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Summary Of Significant Accounting Policies [Line Items]
Decrease to deferred revenue	$ 1,400,000	1,373,000
Increase to retained earnings	$ 1,400,000	$ (1,088,000)
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		3 years
Lease liability		$ 50,400,000
Right-of use assets		$ 31,200,000
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives		5 years
Lease liability		$ 54,800,000
Right-of use assets		$ 35,600,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Financial Instruments Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Total	$ 268,576	$ 266,245
Money market funds [Member]
Assets
Cash equivalents	40,849	31,802
Agency Bonds [Member]
Assets
Cash equivalents	53,999
Certificate of deposit [Member]
Assets
Cash equivalents	1,543	1,543
Corporate bonds [Member]
Assets
Cash equivalents	11,893
Commercial paper [Member]
Assets
Cash equivalents	56,152
U.S. Treasury securities [Member]
Assets
U.S. Treasury securities	104,140	232,900
Level 1 [Member]
Assets
Total	198,988	264,702
Level 1 [Member] | Money market funds [Member]
Assets
Cash equivalents	40,849	31,802
Level 1 [Member] | Agency Bonds [Member]
Assets
Cash equivalents	53,999
Level 1 [Member] | U.S. Treasury securities [Member]
Assets
U.S. Treasury securities	104,140	232,900
Level 2 [Member]
Assets
Total	69,588	1,543
Level 2 [Member] | Certificate of deposit [Member]
Assets
Cash equivalents	1,543	$ 1,543
Level 2 [Member] | Corporate bonds [Member]
Assets
Cash equivalents	11,893
Level 2 [Member] | Commercial paper [Member]
Assets
Cash equivalents	$ 56,152

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Summary of Guidance Effect under Topic 605 before Adoption of Topic 606 on Condensed Statements of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Collaboration and license revenue	$ 4,031	$ 5,771	$ 7,580	$ 32,486	$ 13,218	$ 8,333	$ 7,822	$ 10,135	$ 49,868	$ 39,508	$ 30,691
Operating expenses									196,023	190,910	129,903
Operating loss									(146,155)	(151,402)	(99,212)
Net loss	$ (38,753)	$ (47,244)	$ (34,060)	$ (20,390)	$ (29,211)	$ (43,282)	$ (44,286)	$ (33,443)	$ (140,447)	$ (150,222)	$ (65,697)
Net loss per share applicable to common stockholders - basic and diluted	$ (1.12)	$ (1.37)	$ (0.99)	$ (0.63)	$ (1.04)	$ (1.54)	$ (1.58)	$ (1.21)	$ (4.13)	$ (5.38)	$ (2.44)
ASU 2014-09 [Member] | Under Topic 605 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Collaboration and license revenue									$ 52,329
Operating expenses									196,023
Operating loss									(143,694)
Net loss									$ (137,986)
Net loss per share applicable to common stockholders - basic and diluted									$ (4.06)
ASU 2014-09 [Member] | Effect of change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Collaboration and license revenue									$ (2,461)
Operating loss									(2,461)
Net loss									$ (2,461)
Net loss per share applicable to common stockholders - basic and diluted									$ (0.07)

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Summary of Guidance Effect under Topic 605 before Adoption of Topic 606 on Condensed Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Receivables from collaborative partner	$ 5,096		$ 13,133
Deferred revenue, current portion	1,428		12,713
Deferred revenue, long-term portion	10,465		10,223
Accumulated deficit	(294,681)		$ (155,607)
ASU 2014-09 [Member] | Under Topic 605 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Receivables from collaborative partner	5,096
Deferred revenue, current portion	8,187
Deferred revenue, long-term portion	2,618
Accumulated deficit	(293,593)
ASU 2014-09 [Member] | Effect of change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Deferred revenue, current portion	(6,759)
Deferred revenue, long-term portion	7,847
Accumulated deficit	$ (1,088)	$ 1,400

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Summary of Guidance Effect under Topic 605 before Adoption of Topic 606 on Condensed Statement of Cash Flows (Detail) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net loss		$ (38,753)	$ (47,244)	$ (34,060)	$ (20,390)	$ (29,211)	$ (43,282)	$ (44,286)	$ (33,443)	$ (140,447)	$ (150,222)	$ (65,697)
Decrease in deferred revenue in connection with Topic 606 adoption										1,373
Changes in operating assets and liabilities:
Receivables from collaborative partners										8,037	(9,174)	$ 95
Deferred revenue										(11,043)
Cash, cash equivalents and restricted cash at beginning of period	$ 61,333				61,333					61,333
Cash, cash equivalents and restricted cash at end of period		45,496				61,333				45,496	61,333
ASU 2014-09 [Member] | Under Topic 605 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net loss										(137,986)
Changes in operating assets and liabilities:
Receivables from collaborative partners										8,037
Deferred revenue										(12,131)
Cash, cash equivalents and restricted cash at beginning of period	61,333				$ 61,333					61,333
Cash, cash equivalents and restricted cash at end of period		$ 45,496				$ 61,333				45,496	$ 61,333
ASU 2014-09 [Member] | Effect of change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net loss										(2,461)
Decrease in deferred revenue in connection with Topic 606 adoption	$ 1,400									1,373
Changes in operating assets and liabilities:
Deferred revenue										$ 1,088

Cash Equivalents and Marketab_3

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 226,291	$ 233,376
Unrealized Gains	9
Unrealized Losses	(115)	(476)
Estimated Fair Value	226,185	232,900
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	40,849	31,802
Estimated Fair Value	40,849	31,802
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	104,218	233,376
Unrealized Losses	(78)	(476)
Estimated Fair Value	104,140	232,900
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	54,005
Unrealized Gains	9
Unrealized Losses	(15)
Estimated Fair Value	53,999
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	11,897
Unrealized Losses	(4)
Estimated Fair Value	11,893
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	56,171
Unrealized Gains	0
Unrealized Losses	(19)
Estimated Fair Value	56,152
Total cash equivalents and marketable securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	267,140	265,178
Unrealized Gains	9
Unrealized Losses	(115)	(476)
Estimated Fair Value	267,034	264,702
Cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	(40,849)	(31,802)
Estimated Fair Value	$ (40,849)	$ (31,802)

Cash Equivalents and Marketab_4

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Available For Sale Securities Debt Maturities [Abstract]
Amortized Cost, In one year or less	$ 226,291
Amortized Cost Basis	226,291	$ 233,376
Estimated Fair Value, In one year or less	226,185
Total marketable securities, Estimated Fair Value	$ 226,185	$ 232,900

Cash Equivalents and Marketab_5

Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018USD ($)
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on marketable securities	$ 100,000
Sales of available-for-sale securities	$ 0
Cash equivalents and marketable securities average maturity period	4 months
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and marketable securities maturity period	10 months

Property and Equipment - Compon

Property and Equipment - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 45,125	$ 42,443
Less: accumulated depreciation and amortization	(16,407)	(11,681)
Property and equipment, net	28,718	30,762
Computer equipment and software [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,403	1,892
Furniture and fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	968	947
Laboratory equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	19,579	17,429
Leasehold improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 22,175	$ 22,175

Property and Equipment - Additi

Property and Equipment - Additional Information (Detail) - New Lease [Member] $ in Millions	Dec. 31, 2017USD ($)
Property Plant And Equipment [Line Items]
Leasehold improvements	$ 22.2
Incentive to lessee	$ 14.4

Other Accrued Liabilities - Sch

Other Accrued Liabilities - Schedule of Other Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Clinical development	$ 10,513	$ 12,580
Manufacturing	1,104	2,835
Trade payable	3,381	3,995
Unpaid leasehold improvements		7,742
Other	350	367
Total accrued liabilities	$ 15,348	$ 27,519

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	Sep. 23, 2013	Jan. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Authorized capital stock issuable			110,000,000
Common stock, par value			$ 0.001	$ 0.001
Common stock, shares authorized			100,000,000	100,000,000
Preferred stock, par value			$ 0.001	$ 0.001
Preferred stock, shares authorized			10,000,000	10,000,000
Preferred stock, shares outstanding			0	0
Common stock, shares outstanding			34,745,721	28,178,639
Issuance of stock, Shares		5,897,435
Gross proceeds from issuance of common stocks		$ 115,000	$ 115,000
Proceeds from public offering of common stock, net of issuance costs		$ 107,600	$ 107,612
Additional awards granted	0
Stock options granted to a stockholder owning more than 10%, exercise price percentage			110.00%
Stock options granted, expiration term			10 years
Options vesting period			4 years
Weighted-average grant-date fair value per share of stock options granted			$ 10.85	$ 25.78	$ 27.95
Total intrinsic value of options exercised			$ 2,400	$ 5,400	$ 30,800
Total unrecognized compensation expense			$ 27,900
Unrecognized compensation expense expected to recognize, weighted-average period			2 years 6 months
Stock-based compensation expense			$ 17,300	19,700	11,400
Expected dividend yield			0.00%
Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense			$ 11,600	14,000	20,900
Total fair value of share vesting			$ 6,200	30,800	33,200
Employees and directors [Member] | Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unrecognized compensation expense expected to recognize, weighted-average period			1 year 9 months 18 days
Total unrecognized compensation expense to unvested employee and director RSAs			$ 16,600
2013 Omnibus Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for issuance			3,500,000
Percent of annual increase in common stock available for issuance			4.00%
Number of shares of common stock reserved for future issuance			1,884,387
Prior Plans [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for issuance			1,069,985
2013 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock available for issuance			1,138,877
Percent of annual increase in common stock available for issuance			1.00%
Number of shares of common stock reserved for future issuance			250,000
Unrecognized compensation expense expected to recognize, weighted-average period			4 months 12 days
Stock-based compensation expense			$ 500	$ 500	$ 600
Maximum increase in shares reserved for issuance			300,000
Common stock available for issuance, description			Unless our Board provides otherwise, continuing until the expiration of the ESPP, the total number of shares of common stock available for issuance under the ESPP will automatically increase annually on January 1 by the lesser of (i) 1% of the total number of issued and outstanding shares of common stock as of December 31 of the immediately preceding year, or (ii) 300,000 shares of common stock. As of December 31, 2018, 1,138,877 shares of common stock were available for issuance under the ESPP.
Purchase price per share as a percentage of fair market value of our common stock			85.00%
Shares issued under ESPP			100,735
Total unrecognized compensation expense			$ 200
Underwriters Purchase Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Issuance of stock, Shares		769,230

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Option Activity under Stock Plans and Related Information (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance | shares	3,867,645
Number of Shares, Options granted | shares	858,100
Number of Shares, Options exercised | shares	(328,585)
Number of Shares, Options forfeited | shares	(303,640)
Number of Shares, Options expired | shares	(383,339)
Number of Shares, Ending balance | shares	3,710,181
Number of Shares, Options exercisable | shares	2,195,470
Weighted-Average Exercise Price Per Share, Options beginning balance | $ / shares	$ 30.35
Weighted-Average Exercise Price Per Share, Options granted | $ / shares	17.05
Weighted-Average Exercise Price Per Share, Options exercised | $ / shares	8.18
Weighted-Average Exercise Price Per Share, Options forfeited | $ / shares	34.20
Weighted-Average Exercise Price Per Share, Options expired | $ / shares	35.69
Weighted-Average Exercise Price Per Share, Options ending balance | $ / shares	28.37
Weighted-Average Exercise Price Per Share, Options exercisable | $ / shares	$ 26.95
Weighted-Average Remaining Contractual Terms, Options ending balance	7 years 18 days
Weighted-Average Remaining Contractual Terms, Options exercisable	6 years 10 days
Aggregate Intrinsic Value, Options ending balance | $	$ 740,587
Aggregate Intrinsic Value, Options exercisable | $	$ 740,587

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Restricted Stock Award Activity under Stock Plans and Related Information (Detail) - Restricted Stock Awards [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, beginning balance | shares	803,417
Number of Shares, Granted | shares	715,775
Number of Shares, Vested | shares	(392,136)
Number of Shares, Forfeited | shares	(247,026)
Number of Shares, Unvested, Ending balance | shares	880,030
Weighted-Average Grant-Date Fair Value, Unvested, Beginning balance | $ / shares	$ 40.24
Weighted-Average Grant-Date Fair Value, Granted | $ / shares	17.62
Weighted-Average Grant-Date Fair Value, Vested | $ / shares	34.89
Weighted-Average Grant-Date Fair Value, Forfeited | $ / shares	32.66
Weighted-Average Grant-Date Fair Value, Unvested, Ending balance | $ / shares	$ 26.36

Stockholders' Equity - Schedu_3

Stockholders' Equity - Schedule of Stock-Based Compensation Expenses Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$ 29,468	$ 34,173	$ 32,885
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	15,426	18,285	17,960
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$ 14,042	$ 15,888	$ 14,925

Stockholders' Equity - Schedu_4

Stockholders' Equity - Schedule of Stock Option Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%
Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	2.60%	1.90%	1.30%
Risk-free interest rate, maximum	2.90%	2.20%	1.80%
Expected dividend yield	0.00%	0.00%	0.00%
Minimum [Member] | Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 6 months	5 years 6 months	5 years 6 months
Expected volatility	68.00%	66.00%	69.00%
Maximum [Member] | Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 3 months 18 days	6 years 3 months 18 days	6 years 3 months 18 days
Expected volatility	70.00%	70.00%	74.00%
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 months	6 months	6 months
Risk-free interest rate, minimum	1.40%	1.00%	0.40%
Risk-free interest rate, maximum	2.50%	1.40%	0.60%
Expected dividend yield	0.00%	0.00%	0.00%
ESPP [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	47.00%	42.00%	47.00%
ESPP [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	94.00%	94.00%	57.00%

Earnings per Share - Computatio

Earnings per Share - Computation of Basic and Diluted Net Loss (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (38,753)	$ (47,244)	$ (34,060)	$ (20,390)	$ (29,211)	$ (43,282)	$ (44,286)	$ (33,443)	$ (140,447)	$ (150,222)	$ (65,697)
Denominator:
Denominator for basic loss per share - weighted-average shares									33,976	27,945	26,955
Denominator for diluted loss per share									33,976	27,945	26,955
Net loss per share applicable to common stockholders - basic and diluted	$ (1.12)	$ (1.37)	$ (0.99)	$ (0.63)	$ (1.04)	$ (1.54)	$ (1.58)	$ (1.21)	$ (4.13)	$ (5.38)	$ (2.44)

Earnings per Share - Securities

Earnings per Share - Securities Excluded from Calculation of Diluted Net Loss Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share	4,590	4,729	4,259
Options to purchase common stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share	3,710	3,843	2,981
RSAs [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share	880	886	1,278

License and Collaboration Agr_3

License and Collaboration Agreements - Additional Information (Detail) - USD ($)	Feb. 14, 2015	Sep. 30, 2018	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2014	Apr. 30, 2014	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018	Dec. 19, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized							$ 4,031,000	$ 5,771,000	$ 7,580,000	$ 32,486,000	$ 13,218,000	$ 8,333,000	$ 7,822,000	$ 10,135,000	$ 49,868,000	$ 39,508,000	$ 30,691,000
Issuance of stock, Shares			5,897,435
Aggregate purchase price			$ 107,600,000												107,612,000
Revenue recognized for milestone achieved															12,218,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															(2,461,000)
Bristol-Myers Squibb Company [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront payment	$ 30,000,000
Bristol-Myers Squibb Company [Member] | Cabiralizumab Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															6,900,000
Remaining transaction price recorded in deferred revenue							5,200,000								5,200,000				$ 5,200,000
Received upfront payment					$ 30,000,000
Deferred revenue recognized under collaboration arrangement				$ 11,800,000			5,200,000				11,800,000				5,200,000	11,800,000			5,200,000
Recognized revenue under collaboration arrangement															38,400,000	$ 23,700,000	$ 14,400,000	$ 350,000,000
Non-refundable up-front fee															350,000,000
Adjustments to transaction price															$ 0
License and collaboration agreement entered date															Oct. 14, 2015
Bristol-Myers Squibb Company [Member] | Cabiralizumab Collaboration Agreement [Member] | Milestone Revenue
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized for milestone achieved			25,000,000															$ 0
Bristol-Myers Squibb Company [Member] | Cabiralizumab Collaboration Agreement [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized																			24,800,000
Bristol-Myers Squibb Company [Member] | Cabiralizumab Collaboration Agreement [Member] | ASU 2014-09 [Member] | Actual Costs Incurred as a Percentage of Total Budgeted Costs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															$ 6,600,000
Bristol-Myers Squibb Company [Member] | Immuno-Oncology Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining transaction price recorded in deferred revenue							$ 1,500,000								$ 1,500,000				$ 1,500,000
Research collaboration and license agreement entered month and year															2014-03
Received upfront payment						$ 20,000,000
Research funding obligated to receive over the initial three-year research term															$ 9,500,000
Research agreement, initial term															3 years
Research agreement additional term															2 years
Research agreement additional term description															BMS had the option to extend the research term for two additional one-year periods on a year-by-year basis
Additional research funding for each extension															$ 2,100
Research agreement, initial expiration period															2017-03
Research agreement extended period end																2019-03	2018-03
Research agreement additional term option exercised, description															In each of December 2016 and December 2017, BMS exercised its option to extend the research term for an additional year to March 2018 and March 2019, respectively.
Issuance of stock, Shares															994,352
Price per share							$ 21.16								$ 21.16				$ 21.16
Aggregate purchase price															$ 21,000,000
Deferred revenue recognized under collaboration arrangement							$ 2,400,000								2,400,000				$ 2,400,000
Recognized revenue under collaboration arrangement				5,000,000											6,100,000	$ 12,000,000	$ 7,700,000
Transaction price															36,100,000
Non-refundable up-front fee															20,000,000
Research funding received															13,700,000
Equity premium															2,400,000
Adjustments to transaction price															0
Deferred revenue relating to collaboration agreement				$ 6,300,000			1,500,000				$ 6,300,000				1,500,000	$ 6,300,000			1,500,000
Bristol-Myers Squibb Company [Member] | Immuno-Oncology Collaboration [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															34,600,000
Bristol-Myers Squibb Company [Member] | Immuno-Oncology Collaboration [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															700,000
Bristol-Myers Squibb Company [Member] | Immuno-Oncology Collaboration [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member] | Actual Costs Incurred as a Percentage of Total Budgeted Costs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															6,100,000
Zai Lab [member] | China License and Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															3,300,000
Remaining transaction price recorded in deferred revenue							5,200,000								5,200,000				5,200,000
Non-refundable up-front fee															4,200,000
Adjustments to transaction price															$ 2,200,000
License and collaboration agreement entered month and year															2017-12
Non-refundable and non-creditable upfront fee payment received			5,000,000
Non-refundable and non-creditable upfront fee revenue net of value added tax withholdings			4,200,000
Value added tax withholdings			$ 800,000
Receivables related to collaboration agreement		$ 1,700,000					1,700,000	$ 1,700,000							$ 1,700,000				1,700,000
Milestone payment received		2,000,000
Collaborative arrangement milestone payment, net of value-added tax and other withholdings		$ 300,000
Expected reimbursements							8,800,000								8,800,000				8,800,000
Clinical and regulatory development milestone payments															0
Collaboration And License Agreements Transaction Price															14,700,000
Remaining transaction price							12,900,000								12,900,000				12,900,000
Remaining transaction price will be recorded in deferred revenue when invoiced							8,700,000								8,700,000				8,700,000
Zai Lab [member] | China License and Collaboration Agreement [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Specified development and regulatory milestone payments																				$ 39,000,000
Receivables related to collaboration agreement							10,000,000								10,000,000				10,000,000
Zai Lab [member] | China License and Collaboration Agreement [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized															1,700,000
Non-refundable up-front fee															4,200,000
Receivables related to collaboration agreement							12,500,000								12,500,000				12,500,000
Expected reimbursements							$ 8,300,000								$ 8,300,000				$ 8,300,000

License and Collaboration Agr_4

License and Collaboration Agreements - Summary of Changes in Balances of Contract Assets and Contract Liabilities (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Contract With Customer Asset And Liability [Abstract]
Beginning balance	$ 13,133
Additions	39,262
Deductions	(47,299)
Ending balance	5,096
Beginning balance	21,563
Additions for advance billings	4,727
Deductions for performance obligations satisfied in current period	(12,218)
Deductions for performance obligations satisfied in the prior periods in connection with updates to the measure of progress	(2,179)
Ending balance	$ 11,893

License and Collaboration Agr_5

License and Collaboration Agreements - Additional Information 1 (Detail) - USD ($)	1 Months Ended			3 Months Ended								12 Months Ended
	Jan. 31, 2016	Mar. 31, 2013	Jul. 31, 2010	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized				$ 4,031,000	$ 5,771,000	$ 7,580,000	$ 32,486,000	$ 13,218,000	$ 8,333,000	$ 7,822,000	$ 10,135,000	$ 49,868,000	$ 39,508,000	$ 30,691,000
ASU 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												(2,461,000)
Glaxo Smith Kline LLC [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												0	500,000	1,800,000
Glaxo Smith Kline LLC [Member] | Actual Costs Incurred as a Percentage of Total Budgeted Costs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												$ 0	0	3,200,000
Glaxo Smith Kline LLC [Member] | Respiratory Diseases Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research collaboration and license agreement entered month and year												2012-04
Extend research terms	3 months
Extend research date	2016-07
License agreement termination date			2018-04
Glaxo Smith Kline LLC [Member] | Muscle Diseases Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research collaboration and license agreement entered month and year												2010-07
Glaxo Smith Kline LLC [Member] | Respiratory Diseases and Muscle Diseases Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Adjustments to transaction price												$ 0
Clinical and regulatory development milestone payments												$ 0
UCB Pharma S.A. [Member] | Fibrosis and CNS Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Research collaboration and license agreement entered month and year												2013-03
Adjustments to transaction price												$ 0
Received upfront payment		$ 6,000,000
Technology access fee		6,600,000
Research funding		$ 2,000,000
Evaluation period for initial research activities												2 years
UCB Pharma S.A. [Member] | Fibrosis and CNS Collaboration [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Adjustments to transaction price												$ 600,000
Clinical and regulatory development milestone payments												0
Revenue recognized												0	3,000,000	3,100,000
Technology access fee												6,600,000
Research funding				$ 2,000,000								2,000,000
Transaction price												15,600,000
Non-refundable up-front fee												6,000,000
Recognized target evaluation and selection fees												300,000	300,000	$ 400,000
UCB Pharma S.A. [Member] | Fibrosis and CNS Collaboration [Member] | ASU 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized												600,000
Remaining transaction price recorded in deferred revenue								$ 600,000					$ 600,000
UCB Pharma S.A. [Member] | Fibrosis and CNS Collaboration [Member] | Reagent [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reimbursements costs												$ 1,000,000

Acquired Technologies - Additio

Acquired Technologies - Additional Information (Detail) $ in Thousands	1 Months Ended			12 Months Ended
	May 31, 2017USD ($)	Jul. 31, 2015USD ($)	Dec. 31, 2011USD ($)Installment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expense related to license agreement				$ 156,352	$ 150,908	$ 94,072
Inhibrx [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment	$ 5,000
Upfront fee paid for license and services		$ 10,000
Research and development expense related to license agreement		8,000
Defer and capitalize amount related to prepayment of research and development cost		$ 2,000				1,000	$ 1,000
Termination license agreement effective date				Dec. 27, 2017
Galaxy Biotech, LLC [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license payment			$ 3,000
Number of installments | Installment			2
Milestone payment				$ 9,500	0	2,500
BioWa, Inc. and Lonza Sales AG [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment				$ 1,200	$ 0	$ 0
License agreements expiration period				10 years
BioWa, Inc. and Lonza Sales AG [Member] | Bemarituzumab Programs [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment				$ 24,500
BioWa, Inc. and Lonza Sales AG [Member] | FPA150 Antibody Programs [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Milestone payment				$ 25,400

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 12, 2016	Dec. 31, 2015
Income Tax [Line Items]
Income tax expense (benefit)	$ 0	$ 1,704,000	$ (31,048,000)
Effective corporate tax rate	21.00%	35.00%
Reduction in deferred tax assets related to remeasurement of deferred tax assets and liabilities	$ 27,100,000
Increase (decrease) in valuation allowance	40,200,000	$ 47,400,000
Unrecognized tax benefits	16,660,000	13,584,000	$ 9,400,000	$ 9,403,000	$ 3,432,000
Incurred interest and penalties	$ 0	$ 1,700,000
Ongoing tax examinations by tax authorities	We have no ongoing tax examinations by tax authorities at this time.
Federal tax authority [Member]
Income Tax [Line Items]
Operating losses available for future use	$ 257,000,000
Operating losses available for future use, Expiration date	Dec. 31, 2024
Federal tax authority [Member] | Research and orphan drug credits [Member]
Income Tax [Line Items]
Tax credits available to offset future tax	$ 52,200,000
Tax credit carryforwards, Expiration date	Dec. 31, 2026
State tax authority [Member]
Income Tax [Line Items]
Operating losses available for future use	$ 155,100,000
Operating losses available for future use, Expiration date	Dec. 31, 2028
Tax credits available to offset future tax	$ 21,400,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Income Tax Expense (Benefit) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current tax expense (benefit)
Federal		$ 1,703,000	$ (40,740,000)
State		1,000	(5,340,000)
Total current expense (benefit)		1,704,000	(46,080,000)
Deferred tax expense
Federal			15,032,000
Total deferred tax expense			15,032,000
Income tax expense (benefit)	$ 0	$ 1,704,000	$ (31,048,000)

Income Taxes - Schedule of Inco

Income Taxes - Schedule of Income Tax Expense (Benefit) Differs from Amount Computed by Applying Statutory Federal Income Tax Rate (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Abstract]
Federal statutory income tax	$ (29,495,000)	$ (51,981,000)	$ (33,862,000)
State statutory income tax	1,000	1,000	(3,471,000)
Stock compensation	3,698,000	(4,847,000)	715,000
Nontaxable equity premiums	(85,000)	(168,000)	(248,000)
Change in valuation allowance	38,953,000	41,633,000	12,152,000
Remeasurement of deferred taxes		27,122,000
Research and orphan drug credits	(13,192,000)	(11,029,000)	(8,029,000)
Interest charge, net of federal benefit		1,107,000
Other permanent items	120,000	(134,000)	1,695,000
Income tax expense (benefit)	$ 0	$ 1,704,000	$ (31,048,000)

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Tax Assets Liabilities Net [Abstract]
Net operating loss carryforwards	$ 60,566	$ 39,405
Research and orphan drug credits	58,614	42,070
Deferred revenue	2,410	3,701
Stock-based compensation	7,698	7,017
Capitalized license and depreciation basis differences	3,412	892
Reserves, accruals and tenant improvement allowances	5,639	5,738
Total deferred tax assets	138,339	98,823
Less: valuation allowance	(134,492)	(94,315)
Net deferred tax assets	3,847	4,508
Capitalized license and depreciation basis differences	(3,396)	(4,069)
Prepaid expenses	(451)	(439)
Total deferred tax liabilities	(3,847)	(4,508)
Total net deferred tax assets	$ 0	$ 0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Beginning Balance	$ 13,584	$ 9,400	$ 3,432
Additions for prior year tax positions	622	691	4,394
Additions for current year tax positions	2,454	3,490	1,577
Ending Balance	$ 16,660	$ 13,584	$ 9,400

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)InstrumentTerm	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jul. 31, 2018
Operating Leased Assets [Line Items]
Lease term	10 years
Lease option additional extend term	5 years
Rent commencement date	Jan. 1, 2018
Percentage of rent reduction in first six months	50.00%
Deferred rent	$ 7.6	$ 5.4
Unamortized leasehold improvement incentive total	$ 12.2	13.6
Additional lease requirements	The new lease required us to deliver an irrevocable standby letter of credit in an amount of $1.5 million to landlord for the period commencing on the effective date of the agreement until at least 60 days after the expiration of the new lease, subject to 50% reduction on the first day of the sixth lease year if certain conditions are met.
Rent expense	$ 5.9	$ 6.9	$ 2.3
Bemarituzumab Programs [Member]
Operating Leased Assets [Line Items]
Lease term				3 years
Number of sequencing instruments | Instrument	4
Number of terms available | Term	2
Deferred Rent [Member]
Operating Leased Assets [Line Items]
Incentive to lessee	$ 14.4
Irrevocable Standby Letter of Credit [Member]
Operating Leased Assets [Line Items]
Letter of credit	$ 1.5

Commitments and Contingencies_2

Commitments and Contingencies - Schedule of Estimated Future Minimum Commitments for Non-cancelable Operating Leases (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Leases Operating [Abstract]
2,019	$ 7,315
2,020	7,564
2,021	7,705
2,022	7,787
2,023	8,064
2024 and on	35,166
Total estimated minimum payments	$ 73,601

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2019Employees	Mar. 31, 2019USD ($)
Scenario, Forecast [Member]
Subsequent Event [Line Items]
Pretax charges for severance and other costs related to restructuring | $		$ 2
Subsequent Event [Member]
Subsequent Event [Line Items]
Percentage of company's workforce terminated	20.00%
Number of company's workforce terminated | Employees	41

Selected Quarterly Financial _3

Selected Quarterly Financial Information - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 4,031	$ 5,771	$ 7,580	$ 32,486	$ 13,218	$ 8,333	$ 7,822	$ 10,135	$ 49,868	$ 39,508	$ 30,691
Net loss	$ (38,753)	$ (47,244)	$ (34,060)	$ (20,390)	$ (29,211)	$ (43,282)	$ (44,286)	$ (33,443)	$ (140,447)	$ (150,222)	$ (65,697)
Basic and diluted net loss per common share	$ (1.12)	$ (1.37)	$ (0.99)	$ (0.63)	$ (1.04)	$ (1.54)	$ (1.58)	$ (1.21)	$ (4.13)	$ (5.38)	$ (2.44)