Five Prime Therapeutics (FPRX) 10-K 11 Mar 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Mar. 04, 2016	Jun. 30, 2015
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
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	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		27,807,756
Entity Public Float			$ 527

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 149,971	$ 15,267
Marketable securities	367,495	133,787
Receivable from collaborative partners	4,054	410
Prepaid and other current assets	6,761	1,794
Total current assets	528,281	151,258
Property and equipment, net	4,539	3,794
Deferred tax asset	15,071
Other long-term assets	394	579
Total assets	548,285	155,631
Current liabilities:
Accounts payable	1,894	1,096
Accrued personnel-related expenses	6,878	4,618
Other accrued liabilities	5,882	1,531
Deferred revenue, current portion	17,509	11,938
Deferred rent, current portion	768	632
Income tax payable, current portion	46,437
Total current liabilities	79,368	19,815
Deferred revenue, long-term portion	31,268	48,628
Deferred rent, long-term portion	865	1,514
Income tax payable, long-term portion	3,283
Other long-term liabilities	$ 295	$ 469
Commitments and Contingencies (Note 12)
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized, 27,691,756 issued and 26,116,886 outstanding at December 31, 2015. 21,680,494 shares issued and outstanding at December 31, 2014	$ 26	$ 22
Additional paid-in capital	372,605	274,180
Accumulated other comprehensive income (loss)	(74)	1
Retained earnings (accumulated deficit)	60,649	(188,998)
Total stockholders’ equity	433,206	85,205
Total liabilities and stockholders’ equity	$ 548,285	$ 155,631

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2015	Dec. 31, 2014
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	27,691,756	21,680,494
Common stock, shares outstanding	26,116,886	21,680,494

Statements of Operations

Statements of Operations - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Collaboration revenue	$ 379,801,000	$ 19,231,000	$ 13,791,000
Operating expenses:
Research and development	70,197,000	43,173,000	32,785,000
General and administrative	22,631,000	13,632,000	10,427,000
Total operating expenses	92,828,000	56,805,000	43,212,000
Operating income (loss)	286,973,000	(37,574,000)	(29,421,000)
Interest income	487,000	210,000	62,000
Other (expense) income, net	(3,000)	(60,000)	487,000
Income (loss) before income tax	287,457,000	(37,424,000)	(28,872,000)
Provision for income tax	(37,810,000)	0	0
Net income (loss)	$ 249,647,000	$ (37,424,000)	$ (28,872,000)
Net income (loss) per share attributable to common stockholders:
Basic	$ 9.73	$ (1.79)	$ (5.23)
Diluted	$ 9.23	$ (1.79)	$ (5.23)
Weighted-average shares used to compute net income (loss) per share attributable to common stockholders:
Basic	25,661	20,865	5,523
Diluted	27,035	20,865	5,523

Statements of Comprehensive Inc

Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 249,647	$ (37,424)	$ (28,872)
Other comprehensive loss:
Net unrealized loss on marketable securities, net of tax	(75)	(2)	(4)
Comprehensive income (loss)	$ 249,572	$ (37,426)	$ (28,876)

Statement of Convertible Prefer

Statement of Convertible Preferred Stock and Stockholders' Deficit - USD ($) $ in Thousands	Total	Initial Public Offering [Member]	Research Collaboration [Member]	Convertible preferred stock [Member]	Common Stock [Member]	Common Stock [Member]Initial Public Offering [Member]	Common Stock [Member]Research Collaboration [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]Initial Public Offering [Member]	Additional Paid-in Capital [Member]Research Collaboration [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Beginning Balance at Dec. 31, 2012	$ (115,878)			$ 136,282	$ 1			$ 6,816			$ 7	$ (122,702)
Beginning Balance, Shares at Dec. 31, 2012				9,929,159	1,225,989
Conversion of preferred stock to common stock	136,282			$ (136,282)	$ 10			136,272
Conversion of preferred stock to common stock, Shares				(9,929,159)	9,929,159
Issuance of stock, net of issuance costs		$ 63,849				$ 6			$ 63,843
Issuance of stock, Shares						5,520,000
Issuance of common stock under equity incentive plans and related excess tax benefits	440							440
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares					162,610
Reclassification of warrant liability to additional paid-in capital upon conversion of warrant to purchase Series A convertible preferred stock to warrant to purchase common stock	6							6
Issuance of common stock upon automatic net exercise of warrant	57							57
Issuance of common stock upon automatic net exercise of warrants, Shares					4,376
Stock-based compensation expense related to employee and director option grants	2,067							2,067
Nonemployee stock-based compensation expense	79							79
Other comprehensive loss	(4)										(4)
Net income (loss)	(28,872)											(28,872)
Ending Balance at Dec. 31, 2013	58,026				$ 17			209,580			3	(151,574)
Ending Balance, Shares at Dec. 31, 2013					16,842,134
Issuance of stock, net of issuance costs	40,099		$ 18,629		$ 3		$ 1	40,096		$ 18,628
Issuance of stock, Shares					3,450,000		994,352
Issuance of common stock under equity incentive plans and related excess tax benefits	2,459				$ 1			2,458
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares					393,240
Issuance of common stock upon automatic net exercise of warrants, Shares					768
Stock-based compensation expense related to employee and director option grants	3,284							3,284
Nonemployee stock-based compensation expense	134							134
Other comprehensive loss	(2)										(2)
Net income (loss)	(37,424)											(37,424)
Ending Balance at Dec. 31, 2014	85,205				$ 22			274,180			1	(188,998)
Ending Balance, Shares at Dec. 31, 2014					21,680,494
Issuance of stock, net of issuance costs	78,693				$ 3			78,690
Issuance of stock, Shares					3,829,994
Issuance of common stock under equity incentive plans and related excess tax benefits	8,269				$ 1			8,268
Issuance of common stock under equity incentive plans and related excess tax benefits, Shares					606,398
Stock-based compensation expense related to employee and director option grants	11,201							11,201
Nonemployee stock-based compensation expense	266							266
Other comprehensive loss	(75)										(75)
Net income (loss)	249,647											249,647
Ending Balance at Dec. 31, 2015	$ 433,206				$ 26			$ 372,605			$ (74)	$ 60,649
Ending Balance, Shares at Dec. 31, 2015					26,116,886

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net income (loss)	$ 249,647	$ (37,424)	$ (28,872)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization	1,678	1,552	1,694
Loss on disposal of property and equipment	3	41
Stock-based compensation expense	11,467	3,418	2,146
Amortization of premium on marketable securities	2,025	1,491	432
Revaluation of preferred stock warrant liability			(500)
Excess tax benefits from employee equity incentive plans	3,122
Deferred income taxes	(15,071)
Changes in operating assets and liabilities:
Receivable from collaborative partners	(3,644)	(114)	101
Prepaid, other current assets, and other long-term assets	(4,782)	(579)	(981)
Accounts payable	798	701	(209)
Accrued personnel-related expenses	2,260	1,661	707
Deferred revenue	(11,789)	45,530	280
Deferred rent	(513)	(549)	247
Income tax payable	49,720
Other accrued liabilities and other long-term liabilities	4,177	(463)	(375)
Net cash provided by (used in) operating activities	289,098	15,265	(25,330)
Investing activities
Purchases of marketable securities	(458,058)	(158,674)	(79,776)
Maturities of marketable securities	222,250	90,955	38,403
Purchases of property and equipment	(2,426)	(1,643)	(807)
Net cash (used in) investing activities	(238,234)	(69,362)	(42,180)
Financing activities
Proceeds from issuances of common stock, net of issuance costs	78,693	58,728	63,849
Proceeds from issuances of common stock under equity incentive plans	5,147	2,459	440
Deferred offering costs		16
Payments under capital lease obligation			(9)
Net cash provided by financing activities	83,840	61,203	64,280
Net increase (decrease) in cash and cash equivalents	134,704	7,106	(3,230)
Cash and cash equivalents at beginning of year	15,267	8,161	11,391
Cash and cash equivalents at end of year	$ 149,971	15,267	$ 8,161
Supplemental schedule of noncash financing activities
Accrued and deferred offering costs		$ 144

Business

Business	12 Months Ended
	Dec. 31, 2015
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. Protein therapeutics are antibodies or drugs developed from extracellular proteins or protein fragments that block disease processes, including cancer and inflammatory diseases. We were incorporated in December 2001 in Delaware. Our operations are based in South San Francisco, California and we operate in one segment.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ materially from those estimates. 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 stated at fair value. 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 and reported as a separate component of stockholders’ 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. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Interest on short-term investments is included 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, and does not invest in mortgage-backed securities or mortgage loans. 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. 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, 2015 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 34,821 $ 34,821 $ — $ — U.S. Treasury securities 477,125 477,125 — — Total cash equivalents and marketable securities $ 511,946 $ 511,946 $ - $ — December 31, 2014 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 9,996 $ 9,996 $ — $ — U.S. Treasury securities 130,786 130,786 — — U.S. government agency securities 3,001 — 3,001 — Total cash equivalents and marketable securities $ 143,783 $ 140,782 $ 3,001 $ — 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, 2015, there have been no such impairment losses. Revenue Recognition We recognize revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; our price to the customer is fixed or determinable, and collectability is reasonably assured. The terms of our collaborative research and development agreements include upfront and license fees, research funding, milestone and other contingent payments to us for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. Multiple-Element Revenue Arrangements. Our collaborations primarily represent multiple-element revenue arrangements. To account for these transactions, we determine the elements, or deliverables, included in the arrangement and determine which deliverables are separable for accounting purposes. We consider delivered items to be separable if the delivered items have stand-alone value to the customer. If the delivered items are separable, we allocate arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involve significant judgment, including consideration as to whether each delivered element has standalone value to the customer. We determine the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (VSOE) of selling price, if available, or third party evidence of selling price if VSOE is not available, or our best estimate of selling price, if neither VSOE nor third party evidence is available. Determining the best estimate of selling price for a deliverable requires significant judgment. We use our best estimate of selling price to estimate the selling price for licenses to our proprietary technology since we do not have VSOE or third party evidence of selling price for these deliverables. We recognize consideration allocated to an individual element when all other revenue recognition criteria are met for that element. Our multiple-element revenue arrangements generally include the following: · Exclusive Licenses. The deliverables under our collaboration agreements generally include exclusive licenses to discover, develop, manufacture and commercialize certain compounds. To account for this element of the arrangement, we evaluate whether the exclusive license has standalone value apart from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and other market participants. We recognize arrangement consideration allocated to licenses upon delivery of the license if facts and circumstances indicate that the license has standalone value apart from the undelivered elements, which generally include research and development services. If facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements, we recognize the revenue as a combined unit of accounting. We have determined that some of our exclusive licenses lack standalone value apart from the related research and development services. In those circumstances we recognize collaboration revenue from non-refundable exclusive license fees in the same manner as the undelivered item(s), which is generally the period over which we provide the research and development services. For circumstances in which up-front and license fees are contingently refundable, we defer the recognition of the up-front and license fees until such time that the consideration is considered to be fixed or determinable. · Research and Development Services. The deliverables under our collaboration and license agreements generally include deliverables related to research and development services we perform on behalf of the collaboration partner. As the provision of research and development services is a part of our central operations and we are principally responsible for the performance of these services under the agreements, we recognize revenue on a gross basis for research and development services as we perform those services. Additionally, we recognize research funding related to collaborative research and development efforts as revenue as we perform or deliver the related services in accordance with contract terms as long as we will receive payment for such services upon standard payment terms. Milestone Revenue . Our collaboration and license agreements generally include contingent payments and milestone payments related to specified research, development and regulatory milestones and sales-based milestones. Research, development and regulatory contingent payments and milestone payments are typically payable under our collaborations when our collaborator claims or selects a target or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. At the inception of each arrangement that includes milestone payments, we evaluate whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. We evaluate factors such as the scientific, regulatory, commercial and other risks that we must overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. We have elected to adopt the Financial Accounting Standards Board Accounting Standards Update 2010-17, Revenue Recognition—Milestone Method 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. Expenses we incur related to collaborative research and development agreements approximate the revenue recognized under these agreements. 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 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 enrollment and activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. 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 and stock options. For restricted stock awards, or RSAs, stock-based compensation cost related to employees and directors 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 related to employees and directors 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 are required to estimate forfeitures at the time of grant and revise those estimates in subsequent periods if actual forfeitures differ from those estimates. We use historical data to estimate pre-vesting option forfeitures and record stock-based compensation expense only for those awards that we expect to vest. Options granted to individual service providers who are not employees or directors are accounted for at estimated fair value using the Black-Scholes option-pricing model and are subject to periodic remeasurement over the period during which the services are rendered. 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 bases 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. 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. Recently Issued Accounting Standards In May 2014, the Financial Accounting Standards Board issued Accounting Standards Update 2014-09, Revenue from Contracts with Customers: Topic 606 (ASU 2014-09), to supersede nearly all existing revenue recognition guidance under GAAP. The core principle of ASU 2014-09 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. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than are required under existing GAAP, including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is effective for us in our first quarter of fiscal 2018 using either of two methods: (i) retrospective application of ASU 2014-09 to each prior reporting period presented with the option to elect certain practical expedients as defined within ASU 2014-09; or (ii) retrospective application of ASU 2014-09 with the cumulative effect of initially applying ASU 2014-09 recognized at the date of initial application and providing certain additional disclosures as defined per ASU 2014-09. We are currently evaluating the impact of our pending adoption of ASU 2014-09 on our financial statements. In November 2015, the FASB issued Accounting Standards Update, or ASU, No. 2015-17, Income Taxes (Topic 740): “Balance Sheet Classification of Deferred Taxes” ASU 2015-17 ASU 2015-17 In February 2016, the FASB issued ASU No. 2016-2, Leases. all annual and interim reporting periods thereafter. E

Cash Equivalents and Marketable

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 (in thousands): December 31, 2015 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 34,821 $ — $ — $ 34,821 U.S. Treasury securities 477,239 13 (127 ) 477,125 512,060 13 (127 ) 511,946 Less: cash equivalents (144,470 ) — 19 (144,451 ) Total marketable securities $ 367,590 $ 13 $ (108 ) $ 367,495 December 31, 2014 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 9,996 $ — $ — $ 9,996 U.S. Treasury securities 130,785 18 (17 ) 130,786 U.S. government agency securities 3,001 — — 3,001 143,782 18 (17 ) 143,783 Less: cash equivalents (9,996 ) — — (9,996 ) Total marketable securities $ 133,786 $ 18 $ (17 ) $ 133,787 As of December 31, 2015, 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 $ 477,239 $ 477,125 Total marketable securities $ 477,239 $ 477,125 Our cash equivalents and marketable securities have an average maturity of approximately five months and the longest maturity is nine months. We determined that the gross unrealized losses of $127,000 on our marketable securities as of December 31, 2015 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, 2015. 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, 2015
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and Equipment Property and equipment consist of the following (in thousands): December 31, 2015 2014 Computer equipment and software $ 1,399 $ 1,269 Furniture and fixtures 804 731 Laboratory equipment 11,887 9,978 Leasehold improvements 2,396 2,173 16,486 14,151 Less: accumulated depreciation and amortization (11,947 ) (10,357 ) Property and equipment, net $ 4,539 $ 3,794

Preferred Stock and Common Stoc

Preferred Stock and Common Stock Warrant	12 Months Ended
	Dec. 31, 2015
Equity [Abstract]
Preferred Stock and Common Stock Warrant	5. Preferred Stock and Common Stock Warrant In June 2004, pursuant to the terms of an equipment loan and security agreement, we issued a fully exercisable warrant to the lender for the purchase of 2,304 shares of Series A convertible preferred stock at an exercise price of $12.30 per share. In connection with the completion of our IPO in September 2013, the warrant converted into a warrant to purchase 2,304 shares of common stock at $12.30 per share. We remeasured the fair value of these remaining warrants through the date of the conversion to a common stock warrant and we recorded a $3,000 loss related to the change in fair value as part of other income, net on our statement of operations and reclassified the fair value of $6,000 to permanent equity. The warrant was automatically net exercised for a total of 768 shares on January 26, 2014. In connection with the issuance of Series A convertible preferred stock in January and February 2005, we issued a warrant to purchase 81,300 shares of Series A convertible preferred stock at $12.30 per share to our preferred stock placement agent. During 2007, the warrant was canceled and replaced by the issuance of two warrants for 44,715 and 36,585 shares; all other terms remained unchanged. In connection with the completion of our IPO in September 2013, the warrants were automatically net exercised for a total of 4,376 shares, pursuant to the terms of the warrants. As a result of the net exercises, we recorded an $83,000 gain related to the change in fair value as part of other income, net on our statement of operations and reclassified the fair value of $57,000 to permanent equity. These warrants were remeasured using the intrinsic value of the warrant and the net settlement value based on the $13.00 per share IPO price.

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity (Deficit)	6. Stockholders’ Equity (Deficit) 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. As of December 31, 2015, 1,809,372 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, and nonstatutory stock options may be granted with an exercise price of not less than 85% of the estimated fair value of the common stock on the date of grant. 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 IPO, our Board determined the estimated fair value of our common stock. For all stock options granted after the completion of our IPO in September 2013, the fair value for our underlying common stock is determined using the closing price as reported on The NASDAQ Global Market or The NASDAQ Global Select Market, as applicable, 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. RSAs may be granted for no consideration (other than par value of the shares of stock). The fair value of RSAs is based upon the closing price of our common stock on the date of grant. RSAs generally vest over one and a half to three years and are nontransferable until the awards vest. 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, beginning on January 1, 2014 and 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, 2015, 483,680 shares of common stock were available for issuance under the ESPP. 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 Intristic of Shares Per Share Terms Value (in years) (in thousands) Balance at January 1, 2015 2,684,812 $ 7.85 Options granted 999,088 $ 22.07 Options exercised (519,642 ) $ 7.09 Options forfeited (125,079 ) $ 9.24 Options expired (10,465 ) $ 8.03 Balance at December 31, 2015 3,028,714 $ 12.62 Options exercisable at December 31, 2015 1,464,110 $ 7.52 5.90 $ 49,755 Options vested and expected to vest at December 31, 2015 2,874,728 $ 12.29 7.44 $ 83,983 The weighted-average grant-date fair value per share of stock options granted during the years ended December 31, 2015, 2014 and 2013 was $14.18, $8.39 and $5.05 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2015, 2014 and 2013 was $10.0 million, $4.2 million and $1.1 million, respectively. We recorded stock-based compensation expense related to options granted to employees and directors of approximately $4.5 million, $2.9 million and $2.1 million for the years ended December 31, 2015, 2014 and 2013, respectively. Stock-based compensation expense related to options granted to individual service providers who are not employees or directors was approximately $266,000, $134,000 and $79,000 for the years ended December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015, there was $15.1 million of total unrecognized compensation expense related to nonvested employee and director stock options that we expect to recognize over a weighted-average period of 3.1 years. Restricted Stock Awards 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. We have granted RSAs to certain employees beginning in 2013. The following table summarizes the RSAs activity under our stock plans and related information: RSAs Outstanding Weighted-Average Number Grant-Date of Shares Fair Value Unvested balance at January 1, 2015 24,000 $ 12.18 RSAs granted 1,589,940 $ 19.81 RSAs vested (2,000 ) $ 13.47 RSAs forfeited (37,070 ) $ 19.17 Unvested balance at December 31, 2015 1,574,870 $ 19.71 The total fair value on the date of vesting of RSAs vested in 2015, 2014 and 2013 was $42,020, $54,000, and $0. There were no RSAs granted prior to 2013. As of December 31, 2015, there was $23.4 million of unrecognized compensation cost related to unvested RSAs, net of forfeitures, that we expect to recognize over a weighted-average period of 1.3 years. Employee Stock Purchase Plan 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 67,686 shares under the ESPP in 2015. The compensation expense related to the ESPP was $455,000, $339,000 and $42,000 for the years ended December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015, there was $0.3 million of unrecognized compensation cost related to the ESPP, which we expect to recognize over 4.5 months. Stock-Based Compensation Employee stock-based compensation expense recognized was calculated based on awards ultimately expected to vest and has been reduced for estimated forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Total stock-based compensation expense recognized was as follows: Year Ended December 31, (in thousands) 2015 2014 2013 Research and development $ 6,362 $ 1,761 $ 968 General and administrative 5,105 1,657 1,178 Total $ 11,467 $ 3,418 $ 2,146 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, 2015 2014 2013 2015 2014 2013 Expected term (years) 5.5-6.1 5.3-6.7 5.0-6.1 0.5 0.5 0.5 Expected volatility 71.0-76.0% 85.0% 85.0% 75.0-96.0% 49.0-83.0% 62.0% Risk-free interest rate 1.4-1.9% 1.6-2.0% 0.8-2.0% 0.1-0.3% 0.1% 0.1% 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 in 2013 is based on the average historical volatility of a peer group of public companies over the expected term. Volatility for options granted subsequent to 2013 is based on the average of the historical volatility of our stock price and a peer group of public companies. We selected the peer group on the basis of operational and economic similarity with our principal business operations. 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. Convertible Preferred Stock In connection with the completion of our IPO in September 2013, all outstanding shares of convertible preferred stock converted into 9,929,159 shares of common stock.

Earnings per Share

Earnings per Share	12 Months Ended
	Dec. 31, 2015
Earnings Per Share [Abstract]
Earnings per Share	7. Earnings per Share The computation of basic income (loss) per share is based on the weighted-average number of our common shares outstanding. The computation of diluted income (loss) per share is based on the weighted-average number of our common shares outstanding and dilutive potential common shares, which include principally 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 income (loss) (in thousands, except per share data): Year Ended December 31, 2015 2014 2013 Numerator: Net income (loss) $ 249,647 $ (37,424 ) $ (28,872 ) Denominator: Denominator for basic income (loss) per share - weighted-average shares 25,661 20,865 5,523 Effect of dilutive securities: Equity incentive plans 1,374 — — Denominator for diluted income (loss) per share 27,035 20,865 5,523 Income (loss) per share - Basic $ 9.73 $ (1.79 ) $ (5.23 ) Income (loss) per share - Diluted $ 9.23 $ (1.79 ) $ (5.23 ) We did not include potentially dilutive securities that would have an antidilutive effect. In 2014 and 2013, this consisted of all stock options to purchase common stock, RSAs, preferred stock warrants, common stock warrants and convertible preferred stock. The convertible preferred stock contained certain participation rights. In 2015, the potentially dilutive securities consisted of certain stock options to purchase common stock and RSAs. We excluded the following securities (in thousands) from the calculation of diluted net income (loss) per share as the effect would have been antidilutive: Year Ended December 31, 2015 2014 2013 Convertible preferred stock — — 7,209 Options to purchase common stock and RSAs 195 2,388 2,338 Warrants to purchase convertible preferred stock — — 61 Warrants to purchase common stock — — 1 195 2,388 9,609

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2015
Postemployment Benefits [Abstract]
Employee Benefit Plans	8. Employee Benefit Plans We sponsor a 401(k) plan that stipulates that eligible employees can elect to contribute to the 401(k) plan, subject to certain limitations, up to the lesser of the statutory maximum or 100% of eligible compensation on a pre-tax basis. We pay the administrative costs for the plan. Effective January 1, 2015, we elected to match employee contributions to the 401(k) plan, or the Company Match, as permitted by the plan. During 2015, our 401(k) plan made matching contributions on June 15 and December 15 in an amount equal to 50% of the amount contributed by the employee up to an annual maximum Company Match per employee equal to the lesser of (i) 4% of such employee’s compensation, or (ii) $6,000. During 2015, we delivered the Company Match through the issuance of shares of our common stock. We delivered 17,803 shares of our common stock as the Company Match in 2015 and recorded 401(k) plan Company Match expense of $477,000 for the year ended December 31, 2015.

Collaborative Research and Deve

Collaborative Research and Development Agreements	12 Months Ended
	Dec. 31, 2015
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborative Research and Development Agreements	9. Collaborative Research and Development Agreements Bristol-Myers Squibb Company License and Collaboration Agreement On October 14, 2015, we entered into a license and collaboration agreement, or the FPA008 collaboration agreement, with Bristol-Myers Squibb Company, or BMS, pursuant to which we granted BMS exclusive global rights to develop and commercialize certain colony stimulating factor-1 receptor (CSF1R) antibodies, including our monoclonal CSF1R inhibiting antibody that we refer to as FPA008, 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 FPA008 collaboration agreement, BMS will be 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 future studies, including registration-enabling studies to support approval of FPA008 in pigmented villonodular synovitis, or PVNS, and in combination with our proprietary internal or in-licensed compounds, including in oncology. BMS will be responsible for manufacturing and commercializing each licensed product and we will retain rights to a U.S. co-promotion option. This supersedes the clinical trial collaboration agreement we entered into with BMS in November 2014. We will continue to conduct the current Phase 1a/1b clinical trial to evaluate the safety, tolerability and preliminary efficacy of combining Opdivo ® Pursuant to the FPA008 collaboration agreement, BMS made an upfront payment of $350.0 million to us in December 2015. We applied the FASB Accounting Standards Update, or ASU, No. 2009-13, Multiple-Deliverable Revenue Arrangements Additionally, we will be eligible to receive up to $1.05 billion in development and regulatory milestone payments per anti-CSF1R product for oncology indications and up to $340 million in development and regulatory milestone payments per anti-CSF1R product for non-oncology indications, as well as royalties ranging from the high teens to the low twenties, such royalties to be enhanced in the U.S. in the event that we exercise our co-promotion option. We determined that these contingent payments will not be accounted for under the milestone method of revenue recognition as the events that trigger these payments under the agreement with BMS do not meet the definition of a milestone under ASU 2010-17, Milestone Method of Revenue Recognition Under the superseded clinical trial collaboration agreement, BMS paid us an upfront fee of $30.0 million in December 2014. Initially, the $30.0 million upfront fee was contingently refundable if certain change of control events occurred prior to a specified date. The upfront fee was not considered to be fixed or determinable as of December 31, 2014 and was recorded as deferred revenue as of December 31, 2014. Under the FPA008 collaboration agreement, the $30.0 million upfront fee under the clinical trial collaboration is no longer contingently refundable. Therefore, upon execution of the FPA008 collaboration agreement, the upfront fee became fixed or determinable and we started recognizing revenue ratably, using a cumulative catch up method, over the estimated performance period ending in 2019. We will periodically evaluate the estimated performance period based on the progress made under the collaboration. During 2015, we recognized $6.4 million of revenue relating to the upfront fee. For the year ended December 31, 2015 and 2014, we recognized $359.9 million and $0 million of revenue under the collaboration, respectively. As of December 31, 2015 and 2014, we had deferred revenue relating to the collaboration of $23.6 million and $30.0 million, respectively. Immuno-Oncology Research Collaboration In March 2014, we entered into a research collaboration and license agreement, referred to as the immuno-oncology research collaboration, with BMS, to carry out a research program to (i) discover novel interacting proteins in two undisclosed immune checkpoint pathways, which we refer to as the checkpoint pathways, using our target discovery platform; (ii) further the understanding of target biology with respect to targets in these checkpoint pathways; and (iii) discover and pre-clinically develop compounds suitable for development for human therapeutic uses against targets in these checkpoint pathways. Under the immuno-oncology collaboration, we granted BMS an exclusive, worldwide license to research, develop and commercialize products directed towards certain targets in the checkpoint pathways. BMS will have an option to take exclusive licenses to additional targets we may identify in these checkpoint pathways during the course of the immuno-oncology research collaboration. 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 and expect to receive $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 may extend the research term for two additional one-year periods on a year-by-year basis, 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. We applied the FASB Accounting Standards Update, or ASU, No. 2009-13, Multiple-Deliverable Revenue Arrangements 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 accordance with ASU No. 2010-17 we determined that the remaining contingent payments under the immuno-oncology research collaboration do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments under the collaboration do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events solely depends on BMS’s performance. Any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to the collaboration. If we have no remaining performance obligations under the immuno-oncology research collaboration at the time the contingent payment is triggered, we would recognize the contingent payment as revenue in full upon the triggering event. In connection with 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 to be recognized in the same manner as the $20.0 million up-front payment. For the year ended December 31, 2015 and 2014, we recognized $7.0 million and $6.0 million of revenue under the immuno-oncology research collaboration, respectively. As of December 31, 2015 and 2014, we had deferred revenue relating to the immuno-oncology research collaboration of $16.8 and $19.7 million, respectively. 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. GlaxoSmithKline LLC Respiratory Diseases Collaboration In April 2012, we entered into research collaboration and license agreement, referred to as the respiratory diseases collaboration, with GlaxoSmithKline LLC, or GSK, to identify new therapeutic approaches to treat refractory asthma and chronic obstructive pulmonary disease, or COPD, function with a particular focus on identifying novel protein therapeutics and antibody targets. We conducted six customized cell-based screens of our protein library under this agreement. Under the terms of the agreement, GSK paid us an upfront technology access payment of $7.5 million in April 2012. We applied ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements In April 2014, we amended the agreement with GSK. Pursuant to the original agreement, GSK has an option to elect to include additional screening assays under the research plan. The amendment allowed GSK to terminate any additional screening assay it elects under the research plan within six months of so electing, which termination right lapsed unexercised in October 2014. Concurrent with the amendment, GSK exercised its option and expanded the research plan to include two additional screening assays. In connection with GSK’s exercise of its option, we were entitled to receive up to $1.0 million in additional research funding in eight equal quarterly payments for each additional screening assay, for a total of up to $2.0 million in additional research funding for both additional screening assays, of which we have received $2.0 million as of December 31, 2015. In January 2016, we amended our respiratory diseases collaboration to extend the research term by three months to July 2016 to allow for the conduct of additional activities to validate protein targets we discovered in our screens and to increase the research funding by $0.7 million that GSK is obligated to pay us under our collaboration. We are eligible to receive certain option and selection payments, payments for the achievement of certain development activities, and royalties on the sales of products related to targets GSK selects for exclusive development, if any. We are eligible to receive up to $1.8 million of option and selection payments for each target claimed and selected for further development. In addition, prior to the time GSK exercises its right to obtain an exclusive worldwide license to a protein target, we and GSK will discuss and agree on Track 1 Targets, for which GSK will have sole responsibility for the further development and commercialization of products that incorporate or target the protein targets, and Track 2 Targets, for which we will develop biologics that incorporate or target the protein targets through to clinical proof of mechanism in either a Phase 1 clinical trial or Phase 2 clinical trial. We and GSK will take into consideration each party’s available resources and capabilities at the time in deciding which protein targets will be Track 1 Targets or Track 2 Targets, but subject to each party’s general right to alternate in such selection, with GSK having the right to first select. For each Track 2 Target, we are eligible to receive a $4.0 million milestone payment upon initiation of the first GLP toxicology study, a $6.5 million milestone payment upon the initiation of Phase 1 clinical trial and a $11.0 million milestone payment upon the initiation of Phase 2 clinical trial. We are also eligible to receive a $14.0 million option exercise milestone if GSK exercises its option to develop a Track 2 Target prior to the initiation of Phase 2 clinical trial or a $23.0 million option exercise milestone if GSK exercises after the initiation of Phase 2 clinical trial for the Track 2 Targets. Substantive uncertainty exists at the inception of the agreement as to whether any of these milestones will be achieved because of the numerous variables that may affect our ability to identify targets that GSK would be interested in further evaluating or with respect to which GSK would develop products. In accordance with ASU No. 2010-17, we concluded that these milestones under the agreement with GSK are substantive and will be accounted for under the milestone method of revenue recognition. In accordance with ASU No. 2010-17, we determined that the remaining contingent payments under the agreement with GSK do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments under the agreement with GSK do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events is solely dependent on GSK’s performance. Any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment would be recognized as revenue in full upon the triggering event. In connection with the agreement, GSK purchased 381,693 shares of our Series A-3 convertible preferred stock at a price of $26.20 per share, resulting in net cash proceeds to us of $10.0 million. We determined that the purchase price of $26.20 per share exceeded the estimated fair value of the Series A-3 convertible preferred stock by $3.1 million and, therefore, recorded the $3.1 million as deferred revenue to be recognized in the same manner as the upfront technology access payment. In the years ended December 31, 2015, 2014 and 2013, we received $3.9 million, $3.9 million and $3.4 million, respectively, of research funding and milestones related to all research being performed under the respiratory diseases collaboration. Total revenue recognized under this arrangement was $7.3 million, $6.4 million and $5.6 million for the years ended December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015 and 2014, we had deferred revenue relating to this collaboration agreement of $1.7 million and $4.3 million, respectively. Additionally, GSK is obligated to reimburse us for certain specialized research and development costs associated with the screens under the agreement. As of December 31, 2015 and 2014, the receivable from GSK under the agreement related to such costs was $0.3 million. 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, GSK may terminate this agreement at any time with advance written notice, and either party may terminate this agreement with written notice for the other party’s material breach if such party fails to cure the breach or immediately in the case of failure to comply with certain anti-bribery and anti-corruption policies or upon certain insolvency events. FP-1039 License and Collaboration In March 2011, we entered into a license and collaboration agreement with Human Genome Sciences, Inc., or HGS, which was acquired by GSK in 2012, or the FP-1039 license. Pursuant to the agreement we granted GSK an exclusive license to develop and commercialize our FP-1039 product and other FGFR1 fusion proteins in the United States, the European Union and Canada. Under the terms of the agreement, GSK paid us an upfront license fee of $50.0 million. We received full payment of the $50.0 million upfront license fee in March 2011. We applied ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements Additionally, GSK is obligated to reimburse us for all future research and development costs associated with FP-1039 incurred by us in the conduct of research and development activities on behalf of GSK. At the time we entered into the FP-1039 license, we agreed to perform services for the conduct of the then-concluding Phase 1 clinical trial. We also elected to conduct a Phase 2 clinical trial of FP-1039 in endometrial cancer for which we were reimbursed by GSK. The Phase 2 clinical trial was terminated in January 2012 and we are no longer conducting any activities with respect to this trial. Additionally, GSK is obligated to pay us for the costs of other FP-1039 related research and development activities we elect to undertake on behalf of GSK. Revenue from GSK related to these development costs associated with FP-1039 is recognized as we incur these costs. For each of years ended December 31, 2015, 2014 and 2013, we recognized $0.1 million in revenue from GSK related to development costs associated with FP-1039. In March 2016, GSK delivered to us written notice of termination of the FP-1039 license for convenience. Pursuant to the terms of the FP-1039 license, termination of the FP-1039 license will become effective on September 5, 2016, which is 180 days after GSK’s notice of termination. Pursuant to the terms of the FP-1039 license, GSK will continue to conduct and fund the Phase 1b clinical trial that GSK is currently conducting until September 5, 2016. At our election, GSK will either: (A) transfer the conduct of the Phase 1b clinical trial to us, provided that GSK would continue to bear all costs and expenses incurred in connection with the conduct of the Phase 1b clinical trial until the earlier of the completion of the trial or March 4, 2017, which is 180 days after the effective date of the termination of the FP-1039 license; or (B) orderly wind down the conduct of the Phase 1b clinical trial at GSK’s expense. Muscle Diseases Collaboration In July 2010, we entered into a research collaboration and license agreement, or the muscle diseases collaboration with GSK, to identify potential drug targets and drug candidates to treat skeletal muscle diseases. Under the terms of the agreement, we received an upfront technology access payment of $7.0 million in August 2010. The $7.0 million upfront technology access payment was recorded as deferred revenue, which we initially began recognizing over the initial three-year research period under the agreement. We fully recognized the deferred revenue related to this agreement in 2014 as we completed our obligation to provide research services. In May 2011, we amended the agreement to expand the research plan in scope and duration to include an additional cell-based screen and an in vivo We were eligible to receive certain option and selection payments related to targets identified in the collaboration. We are also eligible to receive payments for the achievement of certain development activities and royalties on the sales of products related to targets GSK selected for exclusive development. We were eligible to receive up to $1.8 million of option and selection payments per target when GSK claimed and selected a target for further development. In accordance with ASU No. 2010-17, we concluded that these payments under the agreement with GSK were substantive and accounted for these milestones under the milestone method of revenue recognition. In accordance with ASU No. 2010-17, we determined that the remaining contingent payments under the agreement with GSK do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments under the agreement with GSK do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events is solely dependent on GSK’s performance. Any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If there are no remaining performance obligations under the arrangement at the time the contingent payment is triggered, the contingent payment would be recognized as revenue in full upon the triggering event. In connection with the agreement, GSK purchased 329,597 shares of our Series A-2 convertible preferred stock at a price of $22.76 per share, resulting in net cash proceeds to us of $7.5 million. We determined that the purchase price of $22.76 per share exceeded the estimated fair value of the Series A-2 convertible preferred stock by $3.0 million and, therefore, recorded the $3.0 million as revenue in the same manner as the upfront technology access payment. In December 2012, GSK selected a protein therapeutic target for further evaluation. The related selection fee of $0.3 million was received in 2013. In September 2013, we and GSK entered into an agreement to extend the evaluation period for this protein therapeutic target by approximately eight months. In connection with the extension of the evaluation period, GSK paid a $0.2 million extension fee, which had been fully recognized in revenue over the eight-month extension period in 2014. In October 2013, GSK exercised its right to reserve for further evaluation several protein therapeutic targets for muscle diseases that we discovered pursuant to this agreement with GSK. In connection with reserving these targets for further evaluation, GSK paid us a selection fee of $0.3 million in 2013. In September, 2014, GSK exercised its option to license an undisclosed muscle disease target that we identified. We granted GSK an exclusive, worldwide license to products containing or directed to the target. We received a payment of $1.5 million in connection with the option exercise Total revenue recognized under this arrangement was $0, $3.4 million and $5.8 million for the years ended December 31, 2015, 2014 and 2013. As of December 31, 2014, the deferred revenue related to this agreement had been fully recognized as we completed our obligation to provide research services. 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, GSK may terminate this agreement at any time with advance written notice, and either party may terminate this agreement with written notice for the other party’s material breach if such party fails to cure the breach or upon certain insolvency events. UCB Pharma S.A. In March 2013, we and UCB Pharma, S.A., or UCB, entered into a research collaboration and license agreement to identify potential biologics targets and therapeutics in the areas of fibrosis-related immunologic diseases and central nervous system disorders. We conducted five customized cell-based and in vivo We applied ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements, in evaluating the appropriate accounting for this agreement. In accordance with this guidance, we concluded that we should account for the arrangement as a single unit of accounting and recognize the arrangement consideration in the same manner as the final deliverable, which is research service. Under the terms of the agreement, UCB paid us an upfront payment of $6.0 million in March 2013. In addition, UCB agreed to pay us a $6.6 million technology fee, of which we received $2.2 million in each of the years 2015, 2014 and 2013. UCB also agreed to pay us $2.0 million of research funding during the second and the third years of the research program term, of which we received $1.0 million in each of 2015 and 2014. We recorded the $6.0 million upfront payment, $6.6 million technology access payment and $2.0 million of research funding as deferred revenue, which we will recognize over the initial five-year research period under the agreement. We are eligible to receive certain evaluation and selection fees and contingent payments with respect to each protein target that UCB elects to obtain an exclusive license, and royalties on the sales of products related to such targets, if any. We are eligible to receive up to $0.4 million of target evaluation and selection fees with respect to each target we offer to UCB in the collaboration. Substantive uncertainty exists at the inception of the agreement as to whether any of these fees will be received because of the numerous variables that may affect our ability to identify targets that UCB would be interested in further evaluating or with respect to which UCB would develop products. In accordance with ASU No. 2010-17, we concluded that these fees under the agreement with UCB are substantive and will be accounted for under the milestone method of revenue recognition. During 2015, we received $0.1 million in target evaluation and selection fees. In accordance with ASU No. 2010-17, we determined that the remaining contingent payments under the agreement with UCB do not constitute milestone payments and will not be accounted for under the milestone method of revenue recognition. The events leading to these payments under the agreement with UCB do not meet the definition of a milestone under ASU 2010-17 because the achievement of these events solely depends on UCB’s performance. Any revenue from these contingent payments would be subject to an allocation of arrangement consideration and would be recognized over any remaining period of performance obligations, if any, relating to this arrangement. If we have no remaining performance obligations under the arrangement at the time the contingent payment is triggered, we would recognize the contingent payment as revenue in full upon the triggering event. For the years ended December 31, 2015, 2014 and 2013, we recognized $4.0 million, $3.2 million and $2.2 million of revenue, respectively, under this arrangement. As of December 31, 2015 and 2014, we have deferred revenue relating to this collaboration agreement of $6.7 million and $6.5 million, respectively. Additionally, UCB is obligated to reimburse us for certain specialized research and development costs associated with the screens under the agreement. As of December 31, 2015 and 2014, the receivable from UCB under the agreement related to such costs was $0.3 million and $0.1 million, respectively. 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.

Acquired Technologies

Acquired Technologies	12 Months Ended
	Dec. 31, 2015
Research And Development [Abstract]
Acquired Technologies	10. 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 (fibroblast growth factor receptor 2) monoclonal antibodies. Under the terms of the agreement, we agreed to pay Galaxy an upfront license payment of $3.0 million. The upfront payment was paid 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 is in an early stage of development, and the underlying technology has no alternative future uses, the entire upfront payment was recorded 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. We made milestone payments to Galaxy totaling $2.6 million in 2014. No milestone payment was made during 2015 or prior to 2014. 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 glucocorticoid-induced tumor necrosis factor receptor, or GITR, for therapeutic and diagnostic uses, which we refer to respectively as licensed therapeutic products and licensed diagnostic products, and (b) an exclusive option, or the option, to obtain exclusive, worldwide licenses to multi-specific antibodies developed by Inhibrx that bind to both GITR and other targets, each of which we refer to as a multi-specific product. We can exercise an option with respect to a multi-specific product within a limited period of time after (i) certain activities related to initiating clinical manufacturing of such multi-specific product or (ii) if not earlier exercised, the dosing of the first patient in a Phase 2 clinical trial of such multi-specific product. Pursuant to the agreement, we paid Inhibrx an upfront fee of $10.0 million for the license and for services to be provided by Inhibrx related to a research cell bank in July 2015. Additionally, with respect to each licensed therapeutic product, we will be obligated to pay up to $62.5 million in specified development milestone payments and (i) if such licensed therapeutic product does not receive a Breakthrough Therapy Designation from the U.S. Food and Drug Administration, or FDA, up to $280.0 million in specified regulatory and commercial milestone payments, or (ii) if such licensed therapeutic product receives a Breakthrough Therapy Designation from the FDA, up to $380.0 million in specified regulatory and commercial milestone payments. Inhibrx is also eligible for low double-digit tiered royalties on future product sales. We may pay all or a portion of milestone payments for development and regulatory events in shares of our common stock, subject to certain limitations and conditions. We would be obligated to register for resale under the Securities Act of 1933, as amended, or the Securities Act, any such shares of our common stock. 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

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes We have recorded an income tax expense of $37.8 million as of December 31, 2015. There was no income tax expense recorded for the years ended December 31, 2014 and 2013 as a result of the loss incurred in those years. The components of our income tax expense were as follows: Year Ended December 31, 2015 2014 2013 Current tax expense Federal $ 47,369 $ — $ — State 5,473 — — Total current expense 52,842 — — Deferred tax benefit Federal (15,032 ) — — State - — — Total deferred tax benefit (15,032 ) — — Total tax expense $ 37,810 $ — $ — A reconciliation of the federal statutory income tax rate to our effective income tax rate is as follows (in thousands): Year Ended December 31, 2015 2014 2013 Federal statutory income tax rate $ 100,610 $ (13,098 ) $ (10,105 ) State statutory income tax rate 3,558 — — Nondeductible stock compensation 437 (501 ) 455 Nontaxable equity premiums (443 ) (504 ) (532 ) Deferred tax assets (utilized) not benefitted (62,705 ) 14,075 10,338 Research and development credit (3,846 ) — — Other permanent items 199 28 (156 ) Income tax expense $ 37,810 $ — $ — 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, 2015 2014 Net operating loss carryforwards $ 941 $ 73,722 Research and development credit 819 8,685 Deferred revenue 16,697 2,554 Stock based compensation 4,542 1,384 Capitalized license and depreciation basis differences 3,685 696 Reserves and accruals 3,960 2,599 Total deferred tax assets 30,644 89,640 Less: valuation allowance (15,573 ) (89,640 ) Net deferred tax assets $ 15,071 $ — As a result of the taxable income generated at December 31, 2015, we utilized all of our available federal and state net operating loss carryforwards and substantially all of our federal and state research credits after consideration of any limitations that may result by Internal Revenue Code Section 382 and related state statutes. The ending 2015 deferred tax assets are principally comprised of the federal and state tax effect of temporary differences and state research credits. Our valuation allowance decreased by approximately $74.1 million during 2015 and increased by $17.6 million during 2014. A $15.6 million valuation allowance was maintained against our state deferred tax assets due to the uncertainty surrounding the realization of such assets in the future. On a periodic basis we evaluate the recoverability of deferred tax assets and the need for a valuation allowance. At such time that it is determined that it is more likely than not that the deferred tax assets are realizable or not realizable, the valuation allowance will be adjusted accordingly. The remaining net deferred tax assets of $15.1 million was recognized based on our review of the reversal pattern of these deferred tax assets that may result in a future recovery of taxes paid in the current year. At December 31, 2015, we had approximately $0.9 million and $59,000 of federal and state tax credits available to offset future federal and state tax. The federal research credits expire beginning in 2024. The state research and development tax credits can be carried forward indefinitely. The remaining federal and state credit carryforwards are subject to an annual Section 382 limitation that restricts the amount that can be utilized in any tax year in the future. We had $3.4 million, $2.2 million and $1.8 million of unrecognized tax benefits as of December 31, 2015, 2014 and 2013, respectively. The unrecognized tax benefits are primarily research tax credits for all years. Unrecognized tax benefits may change during the next twelve months for items that arise in the ordinary course of business. As of December 31, 2015, approximately $345,000 of the ending uncertain tax balance is expected to decrease in the next 12 months due to an expiration of the statute of limitation that will impact our effected tax rate. As of December 31, 2015 and 2014, we had no accrued interest or penalties related to income taxes, and no such interest and penalties have been incurred through December 31, 2015. A reconciliation of our unrecognized tax benefits for the years ended December 31, 2015, 2014 and 2013 , Unrecognized Income Tax Benefits Balance as of December 31, 2012 $ 1,535 Additions for current year tax positions 27 Additions for current year tax positions 219 Balance as of December 31, 2013 1,781 Additions for prior year tax positions 11 Additions for current year tax positions 445 Balance as of December 31, 2014 2,237 Additions for prior year tax positions 615 Additions for current year tax positions 580 Balance as of December 31, 2015 $ 3,432 We file U.S. and state income tax returns with varying statutes of limitations. The tax years from inception in 2001 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, 2015
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies Operating Leases In March 2010, we entered into office and laboratory facility lease agreements, which we refer to collectively as the lease, for a facility located in South San Francisco, California. The lease enables us to utilize the facility through December 31, 2017, with an option to extend the term for an additional three years. In November 2014, we entered into an amendment to the lease, which we refer to as the lease amendment, to amend certain terms of the lease and to increase the amount of space leased. The lease amendment was effective as of March 1, 2015 and will expire on December 31, 2017, which is coterminous with the expiration of the lease. In addition, the amendment contains a $0.2 million one-time improvement allowance for costs of leasehold improvements from the landlord. The lease and the lease amendment require us to pay rent as well as additional amounts for operating expenses and maintenance. The minimum annual rent under the lease is subject to increases based on stated rental adjustment terms. For financial reporting purposes, rent expense is recognized on a straight-line basis over the term of the leases. Accordingly, rent expense recognized in excess of rent paid is reflected as deferred rent. Deferred rent totaled $1.6 million totaled $0.6 million and $0.7 million as of December 31, 2015 and 2014, respectively, of which $0.3 million and $0.5 million is included in other long-term liabilities Rent expense for the years ended December 31, 2015, 2014 and 2013 was $2.3 million, $1.9 million, and $1.9 million, respectively. Year ending December 31: 2016 $ 3,363 2017 3,461 Total estimated minimum payments $ 6,824 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 the 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.

Selected Quarterly Financial In

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2015
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 2015 2015 2015 2015 (Unaudited) Revenue $ 4,287 $ 6,315 $ 5,858 $ 363,341 (1) Net income (loss) (11,036 ) (11,474 ) (23,971 ) 296,128 Basic net income (loss) per share (0.44 ) (0.45 ) (0.93 ) 11.37 Diluted net income (loss) per share (0.44 ) (0.45 ) (0.93 ) 10.63 (1) Includes the $350.0 million upfront license fee from the BMS FPA008 collaboration agreement. See Note 9. Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2014 2014 2014 2014 (Unaudited) Revenue $ 3,546 $ 4,981 $ 6,059 $ 4,645 Net loss (8,644 ) (9,866 ) (7,088 ) (11,826 ) Basic and diluted net loss per share (0.46 ) (0.46 ) (0.33 ) (0.55 ) Basic and diluted net income (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 income (loss) per share amounts.

Summary of Significant Accoun21

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Actual results could differ materially from those estimates.
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 stated at fair value.
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 and reported as a separate component of stockholders’ 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. The amortized cost of securities is adjusted for amortization of premiums and accretion of discounts to maturity. Interest on short-term investments is included 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, and does not invest in mortgage-backed securities or mortgage loans. 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. 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, 2015 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 34,821 $ 34,821 $ — $ — U.S. Treasury securities 477,125 477,125 — — Total cash equivalents and marketable securities $ 511,946 $ 511,946 $ - $ — December 31, 2014 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 9,996 $ 9,996 $ — $ — U.S. Treasury securities 130,786 130,786 — — U.S. government agency securities 3,001 — 3,001 — Total cash equivalents and marketable securities $ 143,783 $ 140,782 $ 3,001 $ —
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, 2015, there have been no such impairment losses.
Revenue Recognition	Revenue Recognition We recognize revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; transfer of technology has been completed or services have been rendered; our price to the customer is fixed or determinable, and collectability is reasonably assured. The terms of our collaborative research and development agreements include upfront and license fees, research funding, milestone and other contingent payments to us for the achievement of defined collaboration objectives and certain preclinical, clinical, regulatory and sales-based events, as well as royalties on sales of any commercialized products. Multiple-Element Revenue Arrangements. Our collaborations primarily represent multiple-element revenue arrangements. To account for these transactions, we determine the elements, or deliverables, included in the arrangement and determine which deliverables are separable for accounting purposes. We consider delivered items to be separable if the delivered items have stand-alone value to the customer. If the delivered items are separable, we allocate arrangement consideration to the various elements based on each element’s relative selling price. The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involve significant judgment, including consideration as to whether each delivered element has standalone value to the customer. We determine the estimated selling price for deliverables within each agreement using vendor-specific objective evidence (VSOE) of selling price, if available, or third party evidence of selling price if VSOE is not available, or our best estimate of selling price, if neither VSOE nor third party evidence is available. Determining the best estimate of selling price for a deliverable requires significant judgment. We use our best estimate of selling price to estimate the selling price for licenses to our proprietary technology since we do not have VSOE or third party evidence of selling price for these deliverables. We recognize consideration allocated to an individual element when all other revenue recognition criteria are met for that element. Our multiple-element revenue arrangements generally include the following: · Exclusive Licenses. The deliverables under our collaboration agreements generally include exclusive licenses to discover, develop, manufacture and commercialize certain compounds. To account for this element of the arrangement, we evaluate whether the exclusive license has standalone value apart from the undelivered elements to the collaboration partner based on the consideration of the relevant facts and circumstances of each arrangement, including the research and development capabilities of the collaboration partner and other market participants. We recognize arrangement consideration allocated to licenses upon delivery of the license if facts and circumstances indicate that the license has standalone value apart from the undelivered elements, which generally include research and development services. If facts and circumstances indicate that the delivered license does not have standalone value from the undelivered elements, we recognize the revenue as a combined unit of accounting. We have determined that some of our exclusive licenses lack standalone value apart from the related research and development services. In those circumstances we recognize collaboration revenue from non-refundable exclusive license fees in the same manner as the undelivered item(s), which is generally the period over which we provide the research and development services. For circumstances in which up-front and license fees are contingently refundable, we defer the recognition of the up-front and license fees until such time that the consideration is considered to be fixed or determinable. · Research and Development Services. The deliverables under our collaboration and license agreements generally include deliverables related to research and development services we perform on behalf of the collaboration partner. As the provision of research and development services is a part of our central operations and we are principally responsible for the performance of these services under the agreements, we recognize revenue on a gross basis for research and development services as we perform those services. Additionally, we recognize research funding related to collaborative research and development efforts as revenue as we perform or deliver the related services in accordance with contract terms as long as we will receive payment for such services upon standard payment terms. Milestone Revenue . Our collaboration and license agreements generally include contingent payments and milestone payments related to specified research, development and regulatory milestones and sales-based milestones. Research, development and regulatory contingent payments and milestone payments are typically payable under our collaborations when our collaborator claims or selects a target or initiates or advances a covered product candidate in preclinical or clinical development, upon submission for marketing approval of a covered product with regulatory authorities, upon receipt of actual marketing approvals of a covered product or for additional indications, or upon the first commercial sale of a covered product. Sales-based milestones are typically payable when annual sales of a covered product reach specified levels. At the inception of each arrangement that includes milestone payments, we evaluate whether each milestone is substantive and at risk to both parties on the basis of the contingent nature of the milestone. We evaluate factors such as the scientific, regulatory, commercial and other risks that we must overcome to achieve the respective milestone, the level of effort and investment required to achieve the respective milestone and whether the milestone consideration is reasonable relative to all deliverables and payment terms in the arrangement in making this assessment. We have elected to adopt the Financial Accounting Standards Board Accounting Standards Update 2010-17, Revenue Recognition—Milestone Method
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. Expenses we incur related to collaborative research and development agreements approximate the revenue recognized under these agreements. 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 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 enrollment and activity according to the related agreement. We monitor patient enrollment levels and related activity to the extent reasonably possible and adjust estimates accordingly. 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 and stock options. For restricted stock awards, or RSAs, stock-based compensation cost related to employees and directors 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 related to employees and directors 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 are required to estimate forfeitures at the time of grant and revise those estimates in subsequent periods if actual forfeitures differ from those estimates. We use historical data to estimate pre-vesting option forfeitures and record stock-based compensation expense only for those awards that we expect to vest. Options granted to individual service providers who are not employees or directors are accounted for at estimated fair value using the Black-Scholes option-pricing model and are subject to periodic remeasurement over the period during which the services are rendered.
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 bases 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. 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.
Recently Issued Accounting Standards	Recently Issued Accounting Standards In May 2014, the Financial Accounting Standards Board issued Accounting Standards Update 2014-09, Revenue from Contracts with Customers: Topic 606 (ASU 2014-09), to supersede nearly all existing revenue recognition guidance under GAAP. The core principle of ASU 2014-09 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. ASU 2014-09 defines a five-step process to achieve this core principle and, in doing so, it is possible more judgment and estimates may be required within the revenue recognition process than are required under existing GAAP, including identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. ASU 2014-09 is effective for us in our first quarter of fiscal 2018 using either of two methods: (i) retrospective application of ASU 2014-09 to each prior reporting period presented with the option to elect certain practical expedients as defined within ASU 2014-09; or (ii) retrospective application of ASU 2014-09 with the cumulative effect of initially applying ASU 2014-09 recognized at the date of initial application and providing certain additional disclosures as defined per ASU 2014-09. We are currently evaluating the impact of our pending adoption of ASU 2014-09 on our financial statements. In November 2015, the FASB issued Accounting Standards Update, or ASU, No. 2015-17, Income Taxes (Topic 740): “Balance Sheet Classification of Deferred Taxes” ASU 2015-17 ASU 2015-17 In February 2016, the FASB issued ASU No. 2016-2, Leases. all annual and interim reporting periods thereafter. E

Summary of Significant Accoun22

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Accounting Policies [Abstract]
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, 2015 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 34,821 $ 34,821 $ — $ — U.S. Treasury securities 477,125 477,125 — — Total cash equivalents and marketable securities $ 511,946 $ 511,946 $ - $ — December 31, 2014 Basis of Fair Value Measurements Total Level 1 Level 2 Level 3 Assets Money market funds $ 9,996 $ 9,996 $ — $ — U.S. Treasury securities 130,786 130,786 — — U.S. government agency securities 3,001 — 3,001 — Total cash equivalents and marketable securities $ 143,783 $ 140,782 $ 3,001 $ —

Cash Equivalents and Marketab23

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2015
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, 2015 and 2014 (in thousands): December 31, 2015 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 34,821 $ — $ — $ 34,821 U.S. Treasury securities 477,239 13 (127 ) 477,125 512,060 13 (127 ) 511,946 Less: cash equivalents (144,470 ) — 19 (144,451 ) Total marketable securities $ 367,590 $ 13 $ (108 ) $ 367,495 December 31, 2014 Amortized Unrealized Unrealized Estimated Cost Basis Gains Losses Fair Value Money market funds $ 9,996 $ — $ — $ 9,996 U.S. Treasury securities 130,785 18 (17 ) 130,786 U.S. government agency securities 3,001 — — 3,001 143,782 18 (17 ) 143,783 Less: cash equivalents (9,996 ) — — (9,996 ) Total marketable securities $ 133,786 $ 18 $ (17 ) $ 133,787
Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity	As of December 31, 2015, 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 $ 477,239 $ 477,125 Total marketable securities $ 477,239 $ 477,125

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2015
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2015 2014 Computer equipment and software $ 1,399 $ 1,269 Furniture and fixtures 804 731 Laboratory equipment 11,887 9,978 Leasehold improvements 2,396 2,173 16,486 14,151 Less: accumulated depreciation and amortization (11,947 ) (10,357 ) Property and equipment, net $ 4,539 $ 3,794

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2015
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 Intristic of Shares Per Share Terms Value (in years) (in thousands) Balance at January 1, 2015 2,684,812 $ 7.85 Options granted 999,088 $ 22.07 Options exercised (519,642 ) $ 7.09 Options forfeited (125,079 ) $ 9.24 Options expired (10,465 ) $ 8.03 Balance at December 31, 2015 3,028,714 $ 12.62 Options exercisable at December 31, 2015 1,464,110 $ 7.52 5.90 $ 49,755 Options vested and expected to vest at December 31, 2015 2,874,728 $ 12.29 7.44 $ 83,983
Schedule of Restricted Stock Award Activity under Stock Plans and Related Information	The following table summarizes the RSAs activity under our stock plans and related information: RSAs Outstanding Weighted-Average Number Grant-Date of Shares Fair Value Unvested balance at January 1, 2015 24,000 $ 12.18 RSAs granted 1,589,940 $ 19.81 RSAs vested (2,000 ) $ 13.47 RSAs forfeited (37,070 ) $ 19.17 Unvested balance at December 31, 2015 1,574,870 $ 19.71
Schedule of Stock-Based Compensation Expenses Recognized	Total stock-based compensation expense recognized was as follows: Year Ended December 31, (in thousands) 2015 2014 2013 Research and development $ 6,362 $ 1,761 $ 968 General and administrative 5,105 1,657 1,178 Total $ 11,467 $ 3,418 $ 2,146
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, 2015 2014 2013 2015 2014 2013 Expected term (years) 5.5-6.1 5.3-6.7 5.0-6.1 0.5 0.5 0.5 Expected volatility 71.0-76.0% 85.0% 85.0% 75.0-96.0% 49.0-83.0% 62.0% Risk-free interest rate 1.4-1.9% 1.6-2.0% 0.8-2.0% 0.1-0.3% 0.1% 0.1% 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, 2015
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Income (Loss)	The following table sets forth the computation of basic and diluted net income (loss) (in thousands, except per share data): Year Ended December 31, 2015 2014 2013 Numerator: Net income (loss) $ 249,647 $ (37,424 ) $ (28,872 ) Denominator: Denominator for basic income (loss) per share - weighted-average shares 25,661 20,865 5,523 Effect of dilutive securities: Equity incentive plans 1,374 — — Denominator for diluted income (loss) per share 27,035 20,865 5,523 Income (loss) per share - Basic $ 9.73 $ (1.79 ) $ (5.23 ) Income (loss) per share - Diluted $ 9.23 $ (1.79 ) $ (5.23 )
Securities Excluded from Calculation of Diluted Net Income (Loss) Per Share	We excluded the following securities (in thousands) from the calculation of diluted net income (loss) per share as the effect would have been antidilutive: Year Ended December 31, 2015 2014 2013 Convertible preferred stock — — 7,209 Options to purchase common stock and RSAs 195 2,388 2,338 Warrants to purchase convertible preferred stock — — 61 Warrants to purchase common stock — — 1 195 2,388 9,609

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Schedule of Components of Income Tax Expense	The components of our income tax expense were as follows: Year Ended December 31, 2015 2014 2013 Current tax expense Federal $ 47,369 $ — $ — State 5,473 — — Total current expense 52,842 — — Deferred tax benefit Federal (15,032 ) — — State - — — Total deferred tax benefit (15,032 ) — — Total tax expense $ 37,810 $ — $ —
Schedule of Reconciliation of Federal Statutory Income Tax Rate	A reconciliation of the federal statutory income tax rate to our effective income tax rate is as follows (in thousands): Year Ended December 31, 2015 2014 2013 Federal statutory income tax rate $ 100,610 $ (13,098 ) $ (10,105 ) State statutory income tax rate 3,558 — — Nondeductible stock compensation 437 (501 ) 455 Nontaxable equity premiums (443 ) (504 ) (532 ) Deferred tax assets (utilized) not benefitted (62,705 ) 14,075 10,338 Research and development credit (3,846 ) — — Other permanent items 199 28 (156 ) Income tax expense $ 37,810 $ — $ —
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, 2015 2014 Net operating loss carryforwards $ 941 $ 73,722 Research and development credit 819 8,685 Deferred revenue 16,697 2,554 Stock based compensation 4,542 1,384 Capitalized license and depreciation basis differences 3,685 696 Reserves and accruals 3,960 2,599 Total deferred tax assets 30,644 89,640 Less: valuation allowance (15,573 ) (89,640 ) Net deferred tax assets $ 15,071 $ —
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of our unrecognized tax benefits for the years ended December 31, 2015, 2014 and 2013 , Unrecognized Income Tax Benefits Balance as of December 31, 2012 $ 1,535 Additions for current year tax positions 27 Additions for current year tax positions 219 Balance as of December 31, 2013 1,781 Additions for prior year tax positions 11 Additions for current year tax positions 445 Balance as of December 31, 2014 2,237 Additions for prior year tax positions 615 Additions for current year tax positions 580 Balance as of December 31, 2015 $ 3,432

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Commitments And Contingencies Disclosure [Abstract]
Schedule of Estimated Future Minimum Commitments for Operating Leases	The estimated future minimum commitments under our lease are as follows (in thousands): Year ending December 31: 2016 $ 3,363 2017 3,461 Total estimated minimum payments $ 6,824

Selected Quarterly Financial 29

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2015
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 2015 2015 2015 2015 (Unaudited) Revenue $ 4,287 $ 6,315 $ 5,858 $ 363,341 (1) Net income (loss) (11,036 ) (11,474 ) (23,971 ) 296,128 Basic net income (loss) per share (0.44 ) (0.45 ) (0.93 ) 11.37 Diluted net income (loss) per share (0.44 ) (0.45 ) (0.93 ) 10.63 (1) Includes the $350.0 million upfront license fee from the BMS FPA008 collaboration agreement. See Note 9. Quarter Ended March 31, June 30, September 30, December 31, Quarterly Results of Operations 2014 2014 2014 2014 (Unaudited) Revenue $ 3,546 $ 4,981 $ 6,059 $ 4,645 Net loss (8,644 ) (9,866 ) (7,088 ) (11,826 ) Basic and diluted net loss per share (0.46 ) (0.46 ) (0.33 ) (0.55 )

Business - Additional Informati

Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015Segment
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of operating segment	1

Summary of Significant Accoun31

Summary of Significant Accounting Policies - Summary of Financial Instruments Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
U.S. Treasury securities and government agency securities	$ 367,495	$ 133,787
Total cash equivalents and marketable securities	511,946	143,783
Money market funds [Member]
Assets
Money market funds	34,821	9,996
U.S. Treasury securities and government agency securities	34,821	9,996
U.S. Treasury securities [Member]
Assets
U.S. Treasury securities and government agency securities	477,125	130,786
U.S. government agency securities [Member]
Assets
U.S. Treasury securities and government agency securities		3,001
Level 1 [Member]
Assets
Total cash equivalents and marketable securities	511,946	140,782
Level 1 [Member] | Money market funds [Member]
Assets
Money market funds	34,821	9,996
Level 1 [Member] | U.S. Treasury securities [Member]
Assets
U.S. Treasury securities and government agency securities	$ 477,125	130,786
Level 2 [Member]
Assets
Total cash equivalents and marketable securities		3,001
Level 2 [Member] | U.S. government agency securities [Member]
Assets
U.S. Treasury securities and government agency securities		$ 3,001

Summary of Significant Accoun32

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015USD ($)
Summary Of Significant Accounting Policies [Line Items]
Impairment losses on long lived assets	$ 0
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Property and equipment estimated useful lives	5 years

Cash Equivalents and Marketab33

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 367,590	$ 133,786
Unrealized Gains	13	18
Unrealized Losses	(108)	(17)
Estimated Fair Value	367,495	133,787
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	34,821	9,996
Estimated Fair Value	34,821	9,996
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	477,239	130,785
Unrealized Gains	13	18
Unrealized Losses	(127)	(17)
Estimated Fair Value	477,125	130,786
Marketable securities including cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	512,060	143,782
Unrealized Gains	13	18
Unrealized Losses	(127)	(17)
Estimated Fair Value	511,946	143,783
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		3,001
Estimated Fair Value		3,001
Cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	(144,470)	(9,996)
Unrealized Losses	19
Estimated Fair Value	$ (144,451)	$ (9,996)

Cash Equivalents and Marketab34

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, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis	$ 367,590	$ 133,786
Total marketable securities, Estimated Fair Value	367,495	133,787
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, In one year or less	477,239
Amortized Cost Basis	477,239	130,785
Estimated Fair Value, In one year or less	477,125
Total marketable securities, Estimated Fair Value	$ 477,125	$ 130,786

Cash Equivalents and Marketab35

Cash Equivalents and Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015USD ($)
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on marketable securities	$ 127,000
Sales of available-for-sale securities	$ 0
Cash equivalents and marketable securities average maturity period	5 months
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash equivalents and marketable securities maturity period	9 months

Property and Equipment - Compon

Property and Equipment - Components of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 16,486	$ 14,151
Less: accumulated depreciation and amortization	(11,947)	(10,357)
Property and equipment, net	4,539	3,794
Computer equipment and software [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,399	1,269
Furniture and fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	804	731
Laboratory equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	11,887	9,978
Leasehold improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$ 2,396	$ 2,173

Preferred Stock and Common St37

Preferred Stock and Common Stock Warrant - Additional Information (Detail)	1 Months Ended
	Sep. 30, 2013USD ($)$ / sharesshares	Jan. 26, 2014shares	Dec. 31, 2007Warrantshares	Feb. 28, 2005$ / sharesshares	Jun. 30, 2004$ / sharesshares
Class Of Warrant Or Right [Line Items]
Gain (Loss) related the change in fair value of other income | $	$ 83,000
Reclassification of warrant liability to equity | $	$ 6,000
Net number of warrants exercised | shares	4,376	768
Number of warrants | Warrant			2
Reclassification of warrant liability to permanent equity | $	$ 57,000
Fair value price of warrants, per share | $ / shares	$ 13
Warrants one [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares | shares			44,715
Warrants two [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares | shares			36,585
Series A convertible preferred stock [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares | shares	2,304			81,300	2,304
Warrant to purchase preferred stock, per share | $ / shares	$ 12.30			$ 12.30	$ 12.30
Gain (Loss) related the change in fair value of other income | $	$ (3,000)

Stockholders' Equity (Deficit38

Stockholders' Equity (Deficit) - Additional Information (Detail) - USD ($)	Sep. 23, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional awards granted	0
Nonstatutory stock options granted, exercise price percentage		85.00%
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		$ 14.18	$ 8.39	$ 5.05
Total intrinsic value of options exercised		$ 10,000,000	$ 4,200,000	$ 1,100,000
Total unrecognized compensation expense related to nonvested employee and director stock options		$ 15,100,000
Unrecognized compensation expense expected to recognize, weighted-average period		3 years 1 month 6 days
Expected dividend yield		0.00%
Preferred stock converted into common stock		9,929,159
Employees and directors [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 4,500,000	2,900,000	2,100,000
Non employee options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		$ 266,000	134,000	79,000
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 3 months 18 days
Total fair value of share vesting		$ 42,020	54,000	0
Total unrecognized compensation expense to unvested RSAs, net of forfeitures		$ 23,400,000
Restricted Stock Awards [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		1 year 6 months
Restricted Stock Awards [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		3 years
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		1,809,372
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		483,680
Percent of annual increase in common stock available for issuance		1.00%
Number of shares of common stock reserved for future issuance		250,000
Maximum increase in shares reserved for issuance		300,000
Common stock available for issuance, description		Unless our Board provides otherwise, beginning on January 1, 2014 and 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, 2015, 483,680 shares of common stock were available for issuance under the ESPP.
Unrecognized compensation expense expected to recognize, weighted-average period		4 years 6 months
Stock-based compensation expense		$ 455,000	$ 339,000	$ 42,000
Purchase price per share as a percentage of fair market value of our common stock		85.00%
Shares issued under ESPP		67,686
Total unrecognized compensation expense		$ 300,000

Stockholders' Equity (Deficit39

Stockholders' Equity (Deficit) - Schedule of Option Activity under Stock Plans and Related Information (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)$ / sharesshares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance | shares	2,684,812
Number of Shares, Options granted | shares	999,088
Number of Shares, Options exercised | shares	(519,642)
Number of Shares, Options forfeited | shares	(125,079)
Number of Shares, Options expired | shares	(10,465)
Number of Shares, Ending balance | shares	3,028,714
Number of Shares, Options exercisable | shares	1,464,110
Number of Shares, Options vested and expected to vest | shares	2,874,728
Weighted-Average Exercise Price Per Share, Options beginning balance | $ / shares	$ 7.85
Weighted-Average Exercise Price Per Share, Options granted | $ / shares	22.07
Weighted-Average Exercise Price Per Share, Options exercised | $ / shares	7.09
Weighted-Average Exercise Price Per Share, Options forfeited | $ / shares	9.24
Weighted-Average Exercise Price Per Share, Options expired | $ / shares	8.03
Weighted-Average Exercise Price Per Share, Options ending balance | $ / shares	12.62
Weighted-Average Exercise Price Per Share, Options exercisable | $ / shares	7.52
Weighted-Average Exercise Price Per Share, Options vested and expected to vest | $ / shares	$ 12.29
Weighted-Average Remaining Contractual Terms, Options exercisable	5 years 10 months 24 days
Weighted-Average Remaining Contractual Terms, Options vested and expected to vest	7 years 5 months 9 days
Aggregate Intrinsic Value, Options exercisable | $	$ 49,755
Aggregate Intrinsic Value, Options vested and expected to vest | $	$ 83,983

Stockholders' Equity (Deficit40

Stockholders' Equity (Deficit) - Schedule of Restricted Stock Award Activity under Stock Plans and Related Information (Detail) - Restricted Stock Awards [Member]	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance | shares	24,000
Granted, Number of Shares | shares	1,589,940
Vested, Number of Shares | shares	(2,000)
Forfeited, Number of Shares | shares	(37,070)
Number of Shares, Unvested, Ending balance | shares	1,574,870
Shares, Weighted-Average Grant-Date Fair Value balance at January 1, 2015 | $ / shares	$ 12.18
Shares Granted, Weighted-Average Grant-Date Fair Value | $ / shares	19.81
Shares Vested, Weighted-Average Grant-Date Fair Value | $ / shares	13.47
Shares Forfeited, Weighted-Average Grant-Date Fair Value | $ / shares	19.17
Shares, Weighted-Average Grant-Date Fair Value balance at December 31, 2015 | $ / shares	$ 19.71

Stockholders' Equity (Deficit41

Stockholders' Equity (Deficit) - Schedule of Stock-Based Compensation Expenses Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$ 11,467	$ 3,418	$ 2,146
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	6,362	1,761	968
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$ 5,105	$ 1,657	$ 1,178

Stockholders' Equity (Deficit42

Stockholders' Equity (Deficit) - Schedule of Stock Option Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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]
Expected volatility		85.00%	85.00%
Risk-free interest rate, minimum	1.40%	1.60%	0.80%
Risk-free interest rate, maximum	1.90%	2.00%	2.00%
Expected dividend yield	0.00%	0.00%	0.00%
Options [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	5 years 6 months	5 years 3 months 18 days	5 years
Expected volatility	71.00%
Options [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 1 month 6 days	6 years 8 months 12 days	6 years 1 month 6 days
Expected volatility	76.00%
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 months	6 months	6 months
Expected volatility			62.00%
Risk-free interest rate, minimum	0.10%
Risk-free interest rate, maximum	0.30%
Risk-free interest rate		0.10%	0.10%
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	75.00%	49.00%
ESPP [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	96.00%	83.00%

Earnings per Share - Computatio

Earnings per Share - Computation of Basic and Diluted Net Income (Loss) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income (loss)	$ 296,128	$ (23,971)	$ (11,474)	$ (11,036)	$ (11,826)	$ (7,088)	$ (9,866)	$ (8,644)	$ 249,647	$ (37,424)	$ (28,872)
Denominator:
Denominator for basic income (loss) per share - weighted-average shares									25,661	20,865	5,523
Effect of dilutive securities:
Equity incentive plans									1,374
Denominator for diluted income (loss) per share									27,035	20,865	5,523
Income (loss) per share - Basic	$ 11.37	$ (0.93)	$ (0.45)	$ (0.44)					$ 9.73	$ (1.79)	$ (5.23)
Income (loss) per share - Diluted	$ 10.63	$ (0.93)	$ (0.45)	$ (0.44)					$ 9.23	$ (1.79)	$ (5.23)

Earnings per Share - Securities

Earnings per Share - Securities Excluded from Calculation of Diluted Net Income (Loss) Per Share (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share	195	2,388	9,609
Convertible preferred stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share			7,209
Options to purchase common stock and RSAs [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share	195	2,388	2,338
Warrants to purchase convertible preferred stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net income (loss) per share			61
Warrants 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			1

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) - USD ($)	Jan. 02, 2015	Dec. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Eligible employees contribution percentage		100.00%
Defined contribution plan, employer matching contribution, percent of match	50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay	4.00%
Defined contribution plan, employer matching contribution, amount	$ 6,000
Defined contribution plan, employer matching contribution, shares		17,803
Defined contribution plan, company matching contribution expense		$ 477,000

Collaborative Research and De46

Collaborative Research and Development Agreements - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Apr. 30, 2014	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Oct. 14, 2015
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate purchase price					$ 78,693,000	$ 58,728,000	$ 63,849,000
Bristol-Myers Squibb Company [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Recognized revenue under collaboration arrangement					359,900,000	0
Revenue recognized for development and regulatory milestone payments					0
Upfront agreement fee, receivable		$ 30,000,000
Termination of refundable upfront fee					30,000,000
Revenue recognized from upfront fee					6,400,000
Deferred revenue recognized under collaboration arrangement	$ 23,600,000	30,000,000		$ 23,600,000	$ 23,600,000	30,000,000
Bristol-Myers Squibb Company [Member] | FPA008 Collaboration Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License and collaboration agreement entered date					Oct. 14, 2015
Research funding received					$ 200,000
Receivable from collaboration agreement	3,400,000			3,400,000	3,400,000
Received upfront payment	350,000,000			350,000,000
Recognized revenue under collaboration arrangement	350,000,000
Bristol-Myers Squibb Company [Member] | Oncology Indications [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development and regulatory milestone payments								$ 1,050,000,000
Bristol-Myers Squibb Company [Member] | Non-Oncology Indications [Member] | Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development and regulatory milestone payments								$ 340,000,000
Bristol-Myers Squibb Company [Member] | Immuno-oncology collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Received upfront payment			$ 20,000,000
Recognized revenue under collaboration arrangement					7,000,000	6,000,000
Deferred revenue recognized under collaboration arrangement	$ 2,400,000			$ 2,400,000	2,400,000
Research funding expected 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 may extend the research term for two additional one-year periods on a year-by-year basis
Research agreement term					5 years
Research funding payment received					$ 4,100,000	3,400,000
Issuance of stock, Shares					994,352
Price per share	$ 21.16			$ 21.16	$ 21.16
Aggregate purchase price					$ 21,000,000
Deferred revenue relating to collaboration agreement	$ 16,800,000	$ 19,700,000		$ 16,800,000	$ 16,800,000	$ 19,700,000

Collaborative Research and De47

Collaborative Research and Development Agreements - Additional Information 1 (Detail) - USD ($)

1 Months Ended

12 Months Ended

Jan. 31, 2016

Sep. 30, 2014

Apr. 30, 2014

Oct. 31, 2013

Sep. 30, 2013

Apr. 30, 2012

Mar. 31, 2011

Aug. 31, 2010

Dec. 31, 2015

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Dec. 31, 2011

May. 31, 2011

Glaxo Smith Kline LLC [Member] | Subsequent Event [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Increase the research funding

$ 700,000

Extend research terms

3 months

Extend research date

2016-07

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Received upfront payment

$ 7,500,000

Research funding

$ 10,500,000

Maximum additional research funding to be received for each screening assay

$ 1,000,000

Maximum additional research funding to be received for both screening assay

$ 2,000,000

Research funding payment received

$ 2,000,000

Option and selection payments

1,800,000

Deferred revenue recognized under collaboration arrangement

1,700,000

$ 4,300,000

Research funding received

3,900,000

3,900,000

$ 3,400,000

Recognized revenue under collaboration arrangement

7,300,000

6,400,000

5,600,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Research and development [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Collaboration receivable

$ 300,000

300,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Series A-3 convertible preferred stock [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Convertible preferred stock issued

381,693

Convertible preferred stock price per share

$ 26.20

Proceeds from issuance of preferred stock

$ 10,000,000

Deferred revenue recognized under collaboration arrangement

3,100,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | GLP Toxicology Study [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payments

4,000,000

Glaxo Smith Kline LLC [Member] | Phase 1 Clinical Trial, Respiratory Diseases Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payments

6,500,000

Glaxo Smith Kline LLC [Member] | Phase 2 Clinical Trial, Respiratory Diseases Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payments

11,000,000

Eligible potential milestone payment

14,000,000

Glaxo Smith Kline LLC [Member] | Track 2 Target, Respiratory Diseases Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Eligible potential milestone payment

23,000,000

Glaxo Smith Kline LLC [Member] | FP-1039 License and Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Recognized revenue under collaboration arrangement

$ 100,000

100,000

100,000

Collaborative arrangement rights and obligations

At our election, GSK will either: (A) transfer the conduct of the Phase 1b clinical trial to us, provided that GSK would continue to bear all costs and expenses incurred in connection with the conduct of the Phase 1b clinical trial until the earlier of the completion of the trial or March 4, 2017, which is 180 days after the effective date of the termination of the FP-1039 license; or (B) orderly wind down the conduct of the Phase 1b clinical trial at GSK’s expense.

Termination license agreement effective date

Sep. 5, 2016

Notice period of termination of license agreement

180 days

Glaxo Smith Kline LLC [Member] | Muscle Disorders Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Received upfront payment

$ 7,000,000

Research funding

$ 6,300,000

Research funding received

$ 0

0

1,500,000

Recognized revenue under collaboration arrangement

0

$ 3,400,000

$ 5,800,000

Research agreement, initial term

3 years

Milestone payments received

$ 300,000

$ 300,000

Extension fee

$ 200,000

Extended evaluation period

8 months

Proceeds from options exercised

$ 1,500,000

Glaxo Smith Kline LLC [Member] | Muscle Disorders Discovery Collaboration [Member] | Maximum [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Option and selection payments

$ 1,800,000

Glaxo Smith Kline LLC [Member] | Muscle Disorders Discovery Collaboration [Member] | Series A-2 convertible preferred stock [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Convertible preferred stock issued

329,597

Convertible preferred stock price per share

$ 22.76

Proceeds from issuance of preferred stock

$ 7,500,000

Deferred revenue recognized under collaboration arrangement

$ 3,000,000

FP-1039 License and Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Received upfront payment

$ 50,000,000

FP-1039 License and Collaboration [Member] | Multiple-Deliverable Revenue Arrangements [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Recognized revenue under collaboration arrangement

$ 50,000,000

Collaborative Research and De48

Collaborative Research and Development Agreements - Additional Information 2 (Detail) - UCB Pharma S.A. [Member] - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Expected evaluation period to complete initial research activities	2 years
Received upfront payment	$ 6
Technology access fee received		$ 2.2	$ 2.2	$ 2.2
Technology access fee	6.6
Research funding	$ 2
Research funding payment received		1	1
Research agreement term	5 years
Received target evaluation and selection fees		0.1
Recognized revenue under collaboration arrangement		4	3.2	$ 2.2
Deferred revenue relating to collaboration agreement		6.7	6.5
Research and development [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivable		0.3	$ 0.1
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Eligible cash receipt for target evaluation and selection		$ 0.4

Acquired Technologies - Additio

Acquired Technologies - Additional Information (Detail)	Dec. 31, 2015USD ($)	Jul. 31, 2015USD ($)	Dec. 31, 2011USD ($)Installment	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research and development expense related to license agreement				$ 70,197,000	$ 43,173,000	$ 32,785,000
Inhibrx [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront fee paid for license and services		$ 10,000,000
Milestone payment method description				Additionally, with respect to each licensed therapeutic product, we will be obligated to pay up to $62.5 million in specified development milestone payments and (i) if such licensed therapeutic product does not receive a Breakthrough Therapy Designation from the U.S. Food and Drug Administration, or FDA, up to $280.0 million in specified regulatory and commercial milestone payments, or (ii) if such licensed therapeutic product receives a Breakthrough Therapy Designation from the FDA, up to $380.0 million in specified regulatory and commercial milestone payments. Inhibrx is also eligible for low double-digit tiered royalties on future product sales. We may pay all or a portion of milestone payments for development and regulatory events in shares of our common stock, subject to certain limitations and conditions. We would be obligated to register for resale under the Securities Act of 1933, as amended, or the Securities Act, any such shares of our common stock.
Research and development expense related to license agreement		8,000,000
Defer and capitalize amount related to prepayment of research and development cost	$ 1,000,000	2,000,000
Recognized expenses on research and development				$ 1,000,000
Inhibrx [Member] | Maximum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Collaboration Fee, Payable		62,500,000
Inhibrx [Member] | Not Receives Breakthrough Therapy Designation [Member] | Maximum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Estimated regulatory and commercial milestone payments		280,000,000
Inhibrx [Member] | Receives Breakthrough Therapy Designation [Member] | Maximum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Estimated regulatory and commercial milestone payments		$ 380,000,000
Galaxy Biotech, LLC [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Upfront license payment			$ 3,000,000
Number of installments | Installment			2
Milestone payment				$ 0	$ 2,600,000

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Line Items]
Income tax expense	$ 37,810,000	$ 0	$ 0
Increase (decrease) in valuation allowance	(74,100,000)	17,600,000
Valuation allowance	15,573,000	89,640,000
Net deferred tax assets	15,071,000	0
Unrecognized tax benefits	3,432,000	2,237,000	$ 1,781,000	$ 1,535,000
Significant increase (decrease) to uncertain tax balance	345,000
Accrued interest or penalties	0	$ 0
Incurred interest and penalties	$ 0
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]
Tax credits available to offset future tax	$ 900,000
Federal tax authority [Member] | Research and development tax credits [Member]
Income Tax [Line Items]
Tax credit carryforwards, Expiration date	Dec. 31, 2024
State tax authority [Member]
Income Tax [Line Items]
Tax credits available to offset future tax	$ 59,000

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Income Tax Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current tax expense
Federal	$ 47,369,000
State	5,473,000
Total current expense	52,842,000
Deferred tax benefit
Federal	(15,032,000)
Total deferred tax benefit	(15,032,000)
Income tax expense	$ 37,810,000	$ 0	$ 0

Income Taxes - Schedule of Reco

Income Taxes - Schedule of Reconciliation of Federal Statutory Income Tax Rate (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Abstract]
Federal statutory income tax rate	$ 100,610,000	$ (13,098,000)	$ (10,105,000)
State statutory income tax rate	3,558,000
Nondeductible stock compensation	437,000	(501,000)	455,000
Nontaxable equity premiums	(443,000)	(504,000)	(532,000)
Deferred tax assets (utilized) not benefitted	(62,705,000)	14,075,000	10,338,000
Research and development credit	(3,846,000)
Other permanent items	199,000	28,000	(156,000)
Income tax expense	$ 37,810,000	$ 0	$ 0

Income Taxes - Schedule of Defe

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets Liabilities Net [Abstract]
Net operating loss carryforwards	$ 941	$ 73,722
Research and development credit	819	8,685
Deferred revenue	16,697	2,554
Stock based compensation	4,542	1,384
Capitalized license and depreciation basis differences	3,685	696
Reserves and accruals	3,960	2,599
Total deferred tax assets	30,644	89,640
Less: valuation allowance	(15,573)	(89,640)
Net deferred tax assets	$ 15,071	$ 0

Income Taxes - Schedule of Re54

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Beginning Balance	$ 2,237	$ 1,781	$ 1,535
Additions for prior year tax positions	615	11	27
Additions for current year tax positions	580	445	219
Ending Balance	$ 3,432	$ 2,237	$ 1,781

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Mar. 31, 2010	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Leases Operating [Abstract]
Lease option additional extend term		3 years
Lease expiration		Dec. 31, 2017
Allowance for cost of leasehold improvements	$ 0.2
Deferred rent		$ 1.6	$ 2.1
Incentive to lessee		1.9
Unamortized leasehold improvement incentive total		0.6	0.7
Unamortized leasehold improvement incentive included in other long-term liabilities		0.3	0.5
Rent expense		$ 2.3	$ 1.9	$ 1.9

Commitments and Contingencies56

Commitments and Contingencies - Schedule of Estimated Future Minimum Commitments for Operating Leases (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Leases Operating [Abstract]
2,016	$ 3,363
2,017	3,461
Total estimated minimum payments	$ 6,824

Selected Quarterly Financial 57

Selected Quarterly Financial Information - Summary of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 363,341	$ 5,858	$ 6,315	$ 4,287	$ 4,645	$ 6,059	$ 4,981	$ 3,546
Net income (loss)	$ 296,128	$ (23,971)	$ (11,474)	$ (11,036)	$ (11,826)	$ (7,088)	$ (9,866)	$ (8,644)	$ 249,647	$ (37,424)	$ (28,872)
Income (loss) per share - Basic	$ 11.37	$ (0.93)	$ (0.45)	$ (0.44)					$ 9.73	$ (1.79)	$ (5.23)
Income (loss) per share - Diluted	$ 10.63	$ (0.93)	$ (0.45)	$ (0.44)					$ 9.23	$ (1.79)	$ (5.23)
Basic and diluted net loss per share					$ (0.55)	$ (0.33)	$ (0.46)	$ (0.46)

Selected Quarterly Financial 58

Selected Quarterly Financial Information - Summary of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2015
Bristol-Myers Squibb Company [Member] | FPA008 Collaboration Agreement [Member]
Received upfront payment	$ 350	$ 350