Five Prime Therapeutics (FPRX) 10-K 18 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Mar. 11, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	FPRX
Entity Registrant Name	FIVE PRIME THERAPEUTICS INC
Entity Central Index Key	1175505
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,532,719
Entity Public Float			$310

Statement_of_Convertible_Prefe

Statement of Convertible Preferred Stock and Stockholders' Deficit (Parenthetical) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2012
Statement Of Stockholders Equity [Abstract]
Convertible preferred stock issued, price per share	$26.20
Convertible preferred stock issued, issuance costs	$35
Convertible preferred stock issued, fair value adjustment	$3,146

Balance_Sheets

Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$15,267	$8,161
Marketable securities	133,787	67,561
Receivable from collaborative partners	410	296
Prepaid and other current assets	1,794	1,640
Total current assets	151,258	77,658
Property and equipment, net	3,794	3,744
Other long-term assets	579	389
Total assets	155,631	81,791
Current liabilities:
Accounts payable	1,096	348
Accrued personnel-related expenses	4,618	2,957
Other accrued liabilities	1,531	2,056
Deferred revenue, current portion	11,938	7,913
Deferred rent, current portion	632	549
Total current liabilities	19,815	13,823
Deferred revenue, long-term portion	48,628	7,123
Deferred rent, long-term portion	1,514	2,146
Other long-term liabilities	469	673
Commitments and Contingencies (Note 12)
Stockholders’ equity:
Common stock, $0.001 par value; 100,000,000 shares authorized, 21,680,494 and 16,842,134 shares issued and outstanding at December 31, 2014 and 2013, respectively	22	17
Preferred stock, $0.001 par value; 10,000,000 shares authorized; no shares issued and outstanding
Additional paid-in capital	274,180	209,580
Accumulated other comprehensive income	1	3
Accumulated deficit	-188,998	-151,574
Total stockholders’ equity	85,205	58,026
Total liabilities and stockholders’ equity	$155,631	$81,791

Balance_Sheets_Parenthetical

Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	21,680,494	16,842,134
Common stock, shares outstanding	21,680,494	16,842,134
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Statements_of_Operations

Statements of Operations (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration revenue	$19,231	$13,791	$9,983
Operating expenses:
Research and development	43,173	32,785	28,778
General and administrative	13,632	10,427	9,009
Total operating expenses	56,805	43,212	37,787
Loss from operations	-37,574	-29,421	-27,804
Interest income	210	62	88
Other (expense) income, net	-60	487	121
Net loss	($37,424)	($28,872)	($27,595)
Net loss per share attributable to common stockholders:
Basic	($1.79)	($5.23)	($23.05)
Diluted	($1.79)	($5.23)	($23.05)
Weighted-average shares used to compute net loss per share attributable to common stockholders:
Basic	20,865	5,523	1,197
Diluted	20,865	5,523	1,197

Statements_of_Comprehensive_Lo

Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net loss	($37,424)	($28,872)	($27,595)
Other comprehensive loss:
Net unrealized loss on marketable securities	-2	-4	-3
Comprehensive loss	($37,426)	($28,876)	($27,598)

Statement_of_Convertible_Prefe1

Statement of Convertible Preferred Stock and Stockholders' Deficit (USD $)	Total	Initial Public Offering [Member]	Research Collaboration [Member]	Convertible preferred stock [Member]	Convertible preferred stock [Member]	Common Stock [Member]	Common Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
In Thousands, except Share data					Series A3 Convertible Preferred Stock [Member]		Initial Public Offering [Member]	Research Collaboration [Member]		Initial Public Offering [Member]	Research Collaboration [Member]
Beginning Balance at Dec. 31, 2011	($90,106)			$129,463		$1			$4,990			$10	($95,107)
Beginning Balance, Shares at Dec. 31, 2011				9,547,466		1,161,781
Issuance of stock, net of issuance costs					6,819
Issuance of stock, Shares					381,693
Issuance of common stock under equity incentive plans	105								105
Issuance of common stock under equity incentive plans, Shares						64,208
Stock-based compensation expense related to employee and director option grants	1,655								1,655
Nonemployee stock-based compensation expense	66								66
Other comprehensive loss	-3											-3
Net loss	-27,595												-27,595
Ending Balance at Dec. 31, 2012	-115,878			136,282		1			6,816			7	-122,702
Ending 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	440								440
Issuance of common stock under equity incentive plans, 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 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	2,459					1			2,458
Issuance of common stock under equity incentive plans, 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 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

Statements_of_Cash_Flows

Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net loss	($37,424)	($28,872)	($27,595)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation and amortization	1,552	1,694	1,643
Loss (gain) on disposal of property and equipment	41		-5
Stock-based compensation expense	3,418	2,146	1,721
Amortization of premium on marketable securities	1,491	432	538
Revaluation of preferred stock warrant liability		-500	-119
Changes in operating assets and liabilities:
Receivable from collaborative partners	-114	101	449
Prepaid, other current assets, and other long-term assets	-579	-981	372
Accounts payable	701	-209	196
Accrued personnel-related expenses	1,661	707	-19
Payable to collaborative partner			-3,000
Deferred revenue	45,530	280	7,379
Deferred rent	-549	247	457
Other accrued liabilities and other long-term liabilities	-463	-375	-414
Net cash provided by (used in) operating activities	15,265	-25,330	-18,397
Investing activities
Purchases of marketable securities	-158,674	-79,776	-45,419
Maturities of marketable securities	90,955	38,403	64,636
Purchases of property and equipment	-1,643	-807	-737
Change in restricted cash			38
Net cash (used in) provided by investing activities	-69,362	-42,180	18,518
Financing activities
Proceeds from issuances of common stock, net of issuance costs	58,728	63,849
Proceeds from issuances of convertible preferred stock, net of issuance costs)			6,819
Proceeds from issuances of common stock under equity incentive plans	2,459	440	105
Deferred offering costs	16
Payments under capital lease obligation		-9	-15
Net cash provided by financing activities	61,203	64,280	6,909
Net increase (decrease) in cash and cash equivalents	7,106	-3,230	7,030
Cash and cash equivalents at beginning of year	8,161	11,391	4,361
Cash and cash equivalents at end of year	15,267	8,161	11,391
Supplemental schedule of noncash financing activities
Accrued and deferred offering costs	$144

Business

Business	12 Months Ended
	Dec. 31, 2014
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 novel 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_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
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, 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		$	—
			December 31, 2013
					Basis of Fair Value Measurements
			Total		Level 1			Level 2			Level 3
	Assets
	Money market funds		$	6,456	$	6,456		$	—		$	—
	U.S. Treasury securities			18,852		18,852			—			—
	U.S. government agency securities			48,709		—			48,709			—
	Total cash equivalents and marketable securities		$	74,017	$	25,308		$	48,709		$	—
	Prior to the Company’s initial public offering, or IPO, in September 2013, we had outstanding warrants which were classified as a liability and remeasured to fair value each reporting period using the Black-Scholes option-pricing model. Inputs used to determine estimated fair value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining contractual term of the warrants, risk-free interest rates, expected dividends, and the expected volatility of the price of the underlying stock. As of December 31, 2012, the warrants were remeasured assuming volatility of 85%, a remaining term of 2.1 years and a risk free interest rate of approximately 0.3%. In connection with the completion of the Company’s IPO, substantially all of 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. The remaining outstanding warrant to purchase Series A convertible preferred stock 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 common stock warrant was automatically net exercised for a total of 768 shares on January 26, 2014.
	The change in the estimated fair value of the Level 3 preferred stock warrant liability on a recurring basis is summarized below (in thousands):
			Year Ended December 31,
			2014		2013			2012
	Balance, beginning of year		$	—	$	563		$	682
	Change in fair value recorded in other expense, net			—		(500	)		(119	)
	Exercises			—		(57	)		—
	Conversion of preferred stock warrant to common stock			—		(6	)		—
	warrant and reclassification to permanent equity
	Balance, end of year		$	—	$	—		$	563
	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. The carrying value of long-lived assets is reviewed for impairment whenever events or changes in circumstances indicate that the assets may not be recoverable. An impairment loss is recognized 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, 2014, there have been no such impairment losses.
	Preferred Stock Warrant Liability
	Freestanding warrants for shares that are either putable or redeemable are classified as liabilities on the balance sheet at fair value. Therefore, the freestanding warrants that give the holders the right to purchase our convertible preferred stock are liabilities that are recorded at estimated fair value. At the end of each reporting period, changes in fair value during the period are recorded as a component of other income (expense), net.
	We adjusted the liability for changes in the estimated fair value of the warrants until the earlier of the exercise or expiration of the warrants to purchase shares of convertible preferred stock or the completion of a liquidation event, including the completion of an initial public offering, at which time the liabilities were reclassified to stockholders’ equity (deficit).
	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, such that we recognize any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. A milestone is defined as an event that can only be achieved based in whole or in part on either our performance or the occurrence of a specific outcome resulting from our performance for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved. Therefore, a milestone does not include events for which occurrence is contingent solely on the performance of a collaborative partner. To be substantive, a milestone must meet all the following criteria: the consideration receivable upon the achievement of the milestone is commensurate with either our performance after the agreement to achieve the milestone or the enhancement of value of delivered items as a result of a specific outcome resulting from our performance after the agreement to achieve the milestone, the consideration relates solely to past performance, and the consideration is reasonable relative to all of the deliverables and payment terms in the arrangement.
	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 stocks and stock options. For restricted stock awards, 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 prevesting option forfeitures and record stock-based compensation expense only for those awards that are expected 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.
Net Loss Per Share of Common Stock
We compute basic net loss per common share by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. We did not include potentially dilutive securities consisting of options and restricted stock awards, or RSAs, to purchase common stock, preferred stock warrants, common stock warrants and convertible preferred stock in the diluted net loss per common share calculations for all periods presented because the inclusion of such shares would have had an antidilutive effect. The convertible preferred stock contained certain participation rights.
We excluded the following securities (in thousands) from the calculation of diluted net loss per share as the effect would have been antidilutive.
		Year Ended December 31,
		2014		2013			2012
Convertible preferred stock			—		7,209			9,824
Options and RSAs to purchase common stock			2,388		2,338			2,347
Warrants to purchase convertible preferred stock			—		61			87
Warrants to purchase common stock			—		1			—
			2,388		9,609			12,258
Recently Issued Accounting Standards
In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU 2014-09 supersedes the revenue recognition requirements in “Topic 605, Revenue Recognition” and requires entities to recognize revenue in a way that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 is effective retrospectively for annual or interim reporting periods beginning after December 15, 2016, with early application not permitted. We are currently evaluating the effect the adoption of ASU 2014-09 will have on our financial statements.

Cash_Equivalents_and_Marketabl

Cash Equivalents and Marketable Securities	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 (in thousands):
		31-Dec-14
		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
		December 31, 2013
		Amortized			Unrealized		Unrealized			Estimated
		Cost Basis			Gains		Losses			Fair Value
	Money market funds	$	6,456		$	—	$	—		$	6,456
	U.S. Treasury securities		18,848			4		—			18,852
	U.S. government agency securities		48,709			3		(3	)		48,709
			74,013			7		(3	)		74,017
	Less: cash equivalents		(6,456	)		—		—			(6,456	)
	Total marketable securities	$	67,557		$	7	$	(3	)	$	67,561
	As of December 31, 2014, 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	$	129,232		$	129,239
	In one to two years		14,550			14,544
	Total marketable securities	$	143,782		$	143,783
	We determined that the gross unrealized losses of $17,000 on our marketable securities as of December 31, 2014 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, 2014. 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, 2014
Property Plant And Equipment [Abstract]
Property and Equipment	4. Property and Equipment
	Property and equipment consist of the following (in thousands):
		December 31,
		2014			2013
	Computer equipment and software	$	1,269		$	1,097
	Furniture and fixtures		731			694
	Laboratory equipment		9,978			9,596
	Leasehold improvements		2,173			2,173
			14,151			13,560
	Less: accumulated depreciation and amortization		(10,357	)		(9,816	)
	Property and equipment, net	$	3,794		$	3,744

Preferred_Stock_and_Common_Sto

Preferred Stock and Common Stock Warrant	12 Months Ended
	Dec. 31, 2014
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 the Company’s 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.

Convertible_Preferred_Stock

Convertible Preferred Stock	12 Months Ended
	Dec. 31, 2014
Temporary Equity Disclosure [Abstract]
Convertible Preferred Stock	6. 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.

Stockholders_Equity_Deficit

Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stockholders' Equity (Deficit)	7. 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, 2014, 3,358,568 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 two to three years and are nontransferable until the awards are vested.
	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, 2014, 334,561 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, 2014		2,236,997		$	6.09
	Options granted		824,895		$	11.71
	Options exercised		(307,272	)	$	5.47
	Options forfeited		(68,182	)	$	7.7
	Options expired		(1,626	)	$	1.23
	Balance at December 31, 2014		2,684,812		$	7.85
	Options exercisable at December 31, 2014		1,409,621		$	6.13		5.76	$	29,424
	Options vested and expected to vest at December 31, 2014		2,643,893		$	7.81		7.26	$	50,727
	The weighted-average grant-date fair value per share of stock options granted during the years ended December 31, 2014, 2013 and 2012 was $8.39, $5.05 and $4.06 per share, respectively. The total intrinsic value of options exercised during the years ended December 31, 2014, 2013 and 2012 was $4.2 million, $1.1 million and $0.3 million, respectively.
	We recorded stock-based compensation expense to employees and directors of approximately $2.9 million, $2.1 million and $1.7 million for the years ended December 31, 2014, 2013 and 2012, respectively. Stock-based compensation expense related to options granted to individual service providers who are not employees or directors was approximately $134,000, $79,000 and $66,000 for the years ended December 31, 2014, 2013 and 2012, respectively. As of December 31, 2014, there was $8.0 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 3.0 years.
	Restricted Stock Awards
	Restricted stock awards, or RSAs, are share awards that entitle the holder to receive freely tradable shares of our common stock upon vesting and are unforfeitable once fully vested. The fair value of RSAs was based upon the closing sales price of our common stock on the grant date. The Company has granted RSAs to certain employees starting 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, 2014		1,000		$	15.64
	RSAs granted		26,500		$	12.3
	RSAs vested		(3,500	)	$	14.09
	RSAs forfeited		—		$	—
	Unvested balance at December 31, 2014		24,000		$	12.18
	The total fair value on the date of vesting of RSAs vested in 2014 and 2013 was $54,000 and $0. There were no RSAs granted prior to 2013.
	As of December 31, 2014, there was $0.3 million of unrecognized compensation cost related to unvested RSAs that we expect to recognize over a weighted-average period of 2.5 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. No shares were issued under the ESPP in 2013. We issued a total of 83,860 shares under the ESPP in 2014.
	The compensation expense related to the ESPP was $339,000 and $42,000 for the years ended December 31, 2014 and 2013, respectively. There was no compensation expense incurred prior to 2013. As of December 31, 2014, there was $0.1 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:
		Years Ended December 31
	(in thousands)	2014			2013		2012
	Research and development	$	1,761		$	968	$	705
	General and administrative		1,657			1,178		1,016
	Total	$	3,418		$	2,146	$	1,721
	The fair value of each award was estimated 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,
		2014			2013		2012		2014		2013		2012
	Expected term (years)	5.3-6.7			5.0-6.1		5.0-6.1		0.5		0.5			—
	Expected volatility		85.00%			85.00%		85.00%	49.0 - 83.0%			62.00%		—
	Risk-free interest rate	1.6-2.0%			0.8-2.0%		0.6-1.1%			0.10%	0.10%			—
	Expected dividend yield		0.00%			0.00%		0.00%		0.00%		0.00%		—
	The expected term of options granted represents the period of time that options granted are expected to be outstanding and was determined by calculating the midpoint between the date of vesting and the contractual life of each option. The expected term of the ESPP rights is equal to the six-month look-back period. Volatility for options granted is based on the average historical volatility of a peer group of public companies over the expected term. The peer group was selected 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.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
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. Through December 2014, we have not elected to match employee contributions as permitted by the plan. We pay the administrative costs for the plan.

Collaborative_Research_and_Dev

Collaborative Research and Development Agreements	12 Months Ended
	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborative Research and Development Agreements	9. Collaborative Research and Development Agreements GlaxoSmithKline
	Muscle Diseases Collaboration
	In July 2010, we entered into a research collaboration and license agreement, referred to as the muscle diseases collaboration with GlaxoSmithKline LLC, or 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.
	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 screen using our RIPPS technology. Under the amendment, GSK agreed to provide an additional $6.3 million of research funding over a three-year research program term beginning on the date of the expansion. We received $0 million,      $1.5 million and $4.2 million of research funding in the years ended December 31, 2014, 2013 and 2012, respectively, related to research we performed under the muscle diseases collaboration. Due to this amendment, in May 2011 we revised our estimate of our substantive performance period under this collaboration to extend through the end of this additional research term and began recognizing the remaining unamortized portion of the upfront payment over this revised period into May 2014.
	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 $3.4 million for the year ended December 31, 2014 and $5.8 million for the years ended December 31, 2013 and 2012. As of December 31, 2013, we had deferred revenue relating to this collaboration agreement of $1.9 million. 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.
	Respiratory Diseases Collaboration
	In April 2012, we entered into research collaboration and license agreement, referred to as the respiratory diseases collaboration, with 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 initially planned to conduct up to six customized cell-based screens of our protein library under this agreement. The four-year research term will end in April 2016. 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 evaluating the appropriate accounting for this agreement. In accordance with this guidance, we concluded that the arrangement should be accounted for as a single unit of accounting and that the arrangement consideration should be recognized in the same manner as the final deliverable, which is the research service. The $7.5 million upfront technology access payment was recorded as deferred revenue and is being recognized over the initial four-year research period under the agreement. In addition, GSK agreed to pay us $10.5 million of research funding over the research program term.
	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 are entitled to receive up to $1.0 million in additional research funding in 16 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 $1.0 million as of December 31, 2014. In the years ended December 31, 2014, 2013 and 2012, we received $3.9 million, $3.4 million and $1.3 million, respectively, of research funding related to all research being performed under the respiratory diseases 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.
	Total revenue recognized under this arrangement was $6.4 million, $5.6 million and $3.2 million for the years ended December 31, 2014, 2013 and 2012, respectively. As of December 31, 2014 and 2013, we had deferred revenue relating to this collaboration agreement of $4.3 million and $6.8 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, 2014 and 2013, the receivable from GSK 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, 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., which was acquired by GSK in 2012, and which we refer to as 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 for multiple cancers 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. The agreement also calls for tiered double-digit percentage royalty payments on net sales. GSK has exclusive rights to develop and commercialize FP-1039 for all indications in the United States, the European Union and Canada. We have an option to co-promote FP-1039 in the United States and retain development and commercialization rights in territories outside the United States, the European Union and Canada.
	We applied ASU No. 2009-13, Multiple-Deliverable Revenue Arrangements, in evaluating the appropriate accounting for this agreement. In accordance with this guidance, we identified the initial license, associated technology transfer and services for the conduct of the then-concluding FP-1039 Phase 1 clinical trial as substantive deliverables under this agreement. However, since all of the deliverables were fully delivered by December 31, 2011, the $50.0 million upfront license fee associated with the deliverables was entirely recognized as revenue in 2011.
	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 the years ended December 31, 2014, 2013 and 2012, we recognized $0.1 million, $0.1 million and $0.9 million, respectively, in revenue from GSK related to development costs associated with FP-1039.
	GSK is obligated to pay us certain amounts contingent upon the achievement of pre-specified development, regulatory and commercial criteria, which could total approximately $435.0 million. 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 GSK do not meet the definition of a milestone under ASU 2010-17 because the achievement of these milestones is solely dependent on GSK’s performance. Revenue from these contingent payments will be recognized if and when such payments become due, subject to satisfaction of all the criteria necessary to recognize revenue at that time, because we do not have any outstanding performance obligations under this arrangement.
	The agreement will terminate upon the expiration of the royalty terms of any products that result from the collaboration. In addition, GSK may terminate this agreement at any time with advance written notice, and either party may terminate this agreement 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 plan to conduct up to five customized cell-based and in vivo screens of our protein library under this agreement. We currently expect to complete our initial research activities under this agreement by March 2016. Upon the completion of those research activities, UCB has up to a two-year evaluation period during which we may be obligated to perform additional services at the request of UCB.
	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 2014 and 2013. The remaining $2.2 million technology access fee is due on the second anniversary of this agreement. 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 2014. We recorded the $6.0 million upfront payment, $4.4 million technology access payment and $1.0 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.
	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, 2014 and 2013, we recognized $3.2 million and $2.2 million of revenue, respectively, under this arrangement. As of December 31, 2014 and 2013, we have deferred revenue relating to this collaboration agreement of $6.5 million and $6.2 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, 2014 and 2013, the receivable from UCB under the agreement related to such costs was $0.1 million and $0.2 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.
	Bristol-Myers Squibb Company
	Research Collaboration and License Agreement
	In March 2014, we entered into a research collaboration and license agreement, referred to as the immuno-oncology research collaboration, with Bristol-Myers Squibb Company, or 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, in evaluating the appropriate accounting for the immuno-oncology collaboration. In accordance with this guidance, we concluded that we should account for the immuno-oncology research collaboration as a single unit of accounting because the intellectual property delivered to BMS was not considered to have stand-alone value and recognize the immuno-oncology research collaboration consideration in the same manner as the final deliverable, which is research service. The $20.0 million upfront payment was recorded as deferred revenue and is being recognized over the five-year research period under the collaboration. In addition, BMS agreed to pay us $9.5 million of research funding over the initial three-year research program term. We received $3.4 million of research funding in 2014 related to research we performed under the immuno-oncology research collaboration.
	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, Milestone Method of Revenue Recognition, 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, 2014, we recognized $6.0 million of revenue under the immuno-oncology research collaboration. As of December 31, 2014, we had deferred revenue relating to the immuno-oncology research collaboration of $19.7 million.
	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.
	Clinical Trial Collaboration Agreement
	In November 2014, we entered into a clinical trial collaboration agreement with BMS, referred to as the clinical trial collaboration, to evaluate the safety, tolerability and preliminary efficacy of combining our FPA008 antibody with BMS’s nivolumab antibody, which we refer to together as the combined therapy, as a potential treatment option for patients with non-small cell lung cancer, melanoma, head and neck cancer, pancreatic cancer, colorectal cancer and malignant glioma.
	Pursuant to the terms of the agreement, we are responsible for conducting the clinical phase 1a/1b study. In addition, either BMS or we may, by mutual agreement, expand the scope of the clinical trial collaboration to study additional tumor types using the combined therapy.
	Under the terms of the agreement, BMS paid us a one-time fee of $30.0 million in December 2014. If a change of control of FivePrime closes prior to the initial results date, defined as the earlier date that (i) certain dose escalation and pharmacodynamics conditions are met in the planned Phase 1a portion of the first clinical study of the combined therapy; or (ii) the first subject is dosed in the Phase 1b portion of the first clinical study of the combined therapy, then if,
	•	immediately prior to such change of control, the acquirer in such change of control (or any of its affiliates) owns or controls an anti-PD-1 or anti-PD-L1 antagonist that is then in clinical development for use in treating cancer or is then being commercialized for use in treating cancer;
	•	BMS is using commercially reasonable efforts in the performance and fulfillment of its activities under the clinical trial collaboration;
	•	the parties are developing or pursuing the development of the combined therapy under the clinical trial collaboration; and
	•	a change of control of BMS has not occurred,
	we would be obligated to pay to BMS the lesser of (x) $30.0 million or (y) 10% of the aggregate purchase price paid to us or our stockholders at the closing of such change of control (with any contingent consideration being risk-adjusted and discounted).
	We applied the ASU, No. 2009-13, Multiple-Deliverable Revenue Arrangements, in evaluating the appropriate accounting for the clinical trial collaboration. In accordance with this guidance, we concluded that we should account for the clinical trial collaboration as a single unit of accounting because the delivered items did not have stand-alone value and recognize the clinical trial collaboration consideration in the same manner as the final deliverable, which is conducting the clinical phase 1a/1b study. The $30.0 million one-time fee is contingently refundable upon certain change of control events prior to the initial results date. The one-time 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. Once the one-time fee is considered fixed or determinable, we will start recognizing revenue ratably, using a cumulative catch up method, over the estimated performance period of approximately three years. We will periodically evaluate the estimated performance period based on the progress made under the collaboration. No revenue related to this collaboration has been recognized as of December 31, 2014.
	Unless earlier termination by either party, the clinical trial collaboration will continue until the date that is 90 days after the completion of all clinical trials under the agreement, the delivery of all study data by both parties and the completion of all obligations under the clinical trial collaboration. Either party may terminate the agreement with written notice (i) if the other party is in material breach and such breach has not been cured within the applicable cure period, (ii) if either party deems it necessary to protect the safety, health or welfare of the subjects enrolled in a clinical trial or (iii) 90 days following the commencement of a clinical hold.

Acquired_Technologies

Acquired Technologies	12 Months Ended
	Dec. 31, 2014
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 prior to 2014.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes
	No income tax expense was recorded for the years ended December 31, 2014, 2013 and 2012.
	A reconciliation of the federal statutory income tax rate to our effective income tax rate is as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Federal statutory income tax rate	$	(13,098	)	$	(10,105	)	$	(9,658	)
	Nondeductible stock compensation		(501	)		455			386
	Nontaxable equity premiums		(504	)		(532	)		(452	)
	Deferred tax assets not benefitted		14,075			10,338			9,750
	Other permanent items		28			(156	)		(26	)
	Income tax expense	$	—		$	—		$	—
	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,
		2014			2013
	Net operating loss carryforwards	$	73,722		$	59,552
	Research and development credit		8,685			6,564
	Reserves and accruals		7,233			5,936
	Total deferred tax assets		89,640			72,052
	Less: valuation allowance		(89,640	)		(72,052	)
	Net deferred tax assets	$	—		$	—
	Realization of deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. Accordingly, net deferred tax assets have been fully offset by a valuation allowance. Our valuation allowance increased by approximately $17.6 million and $12.1 million during 2014 and 2013, respectively. We have established a full valuation allowance against our deferred tax assets due to the uncertainty surrounding the realization of such assets. We evaluate on a periodic basis 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, the valuation allowance will be reduced.
	At December 31, 2014, we had approximately $179.7 million and $205.9 million of federal and state net operating loss carryforwards, respectively, available to offset future taxable income. The net operating loss carryforwards begin to expire in 2024 for federal and 2015 for state purposes. We also had approximately $6.9 million and $5.6 million of federal and state tax credits, respectively, available to offset future tax. These credits begin to expire in 2023 for federal purposes, and state research and development tax credits can be carried forward indefinitely.
	Utilization of the net operating loss and credit carryforwards may be subject to substantial annual limitation due to ownership change provisions of the Internal Revenue Code of 1986 and similar state provisions. The annual limitation may result in the expiration of net operating losses and credits before utilization. To the extent net operating loss carryforwards, when realized, relate to non-qualified stock option deductions, the resulting benefits will be credited to stockholders’ equity.
	As of December 31, 2014 and 2013, we had no accrued interest or penalties related to income taxes, and no such interest and penalties have been incurred through December 31, 2014. As of December 31, 2014, no significant increases or decreases are expected to our uncertain tax positions within the next 12 months. A reconciliation of our unrecognized tax benefits for the years ended December 31, 2014, 2013 and 2012, is as follows (in thousands):
		Unrecognized
		Income Tax
		Benefits
	Balance as of January 1, 2012	$	1,457
	Additions for current year tax positions		78
	Balance as of December 31, 2012		1,535
	Additions for prior 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
	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, 2014
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 will be effective as of March 1, 2015 and 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 $2.1 million and $2.7 million at December 31, 2014 and 2013, respectively. In addition, the lease contains a $1.7 million incentive in the form of reimbursement or payments from the landlord for a portion of the costs of leasehold improvements we make to the facility. We made these improvements and received the benefit of the $1.7 million incentive in 2010. The assets purchased with the incentive are included in property and equipment, net in the accompanying balance sheets as of December 31, 2014 and 2013, respectively. The incentive is being recognized as a reduction of rental expense on a straight-line basis over the term of the underlying leases. The unamortized leasehold improvement incentive totaled $0.7 million and $0.9 million as of December 31, 2014 and 2013, respectively, of which $0.5 million and $0.7 million is included in other long-term liabilities in the accompanying balance sheets as of December 31, 2014 and 2013, respectively.
	Rent expense for each of the years ended December 31, 2014, 2013 and 2012 was $1.9 million. The estimated future minimum commitments under our lease are as follows (in thousands):
	Year ending December 31:
	2015	$	3,108
	2016		3,363
	2017		3,461
	Total estimated minimum payments	$	9,932
	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_I

Selected Quarterly Financial Information	12 Months Ended
	Dec. 31, 2014
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	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	)
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
	Quarterly Results of Operations	2013			2013			2013			2013
		(Unaudited)
	Revenue	$	2,975		$	3,549		$	3,482		$	3,785
	Net loss		(7,047	)		(7,274	)		(7,234	)		(7,317	)
	Basic and diluted net loss per share		(5.73	)		(5.82	)		(2.74	)		(0.43	)
	Basic and diluted net loss per share is computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share amounts may not equal annual basic and diluted net loss per share amounts.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events
	Follow-on Public Offering
	In January 2015, we closed an underwritten public offering of 3,829,994 shares of our common stock and received net proceeds of $78.7 million, after underwriting discounts, structuring fees and estimated offering expenses.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
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, 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		$	—
			December 31, 2013
					Basis of Fair Value Measurements
			Total		Level 1			Level 2			Level 3
	Assets
	Money market funds		$	6,456	$	6,456		$	—		$	—
	U.S. Treasury securities			18,852		18,852			—			—
	U.S. government agency securities			48,709		—			48,709			—
	Total cash equivalents and marketable securities		$	74,017	$	25,308		$	48,709		$	—
	Prior to the Company’s initial public offering, or IPO, in September 2013, we had outstanding warrants which were classified as a liability and remeasured to fair value each reporting period using the Black-Scholes option-pricing model. Inputs used to determine estimated fair value include the estimated fair value of the underlying stock at the valuation measurement date, the remaining contractual term of the warrants, risk-free interest rates, expected dividends, and the expected volatility of the price of the underlying stock. As of December 31, 2012, the warrants were remeasured assuming volatility of 85%, a remaining term of 2.1 years and a risk free interest rate of approximately 0.3%. In connection with the completion of the Company’s IPO, substantially all of 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. The remaining outstanding warrant to purchase Series A convertible preferred stock 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 common stock warrant was automatically net exercised for a total of 768 shares on January 26, 2014.
	The change in the estimated fair value of the Level 3 preferred stock warrant liability on a recurring basis is summarized below (in thousands):
			Year Ended December 31,
			2014		2013			2012
	Balance, beginning of year		$	—	$	563		$	682
	Change in fair value recorded in other expense, net			—		(500	)		(119	)
	Exercises			—		(57	)		—
	Conversion of preferred stock warrant to common stock			—		(6	)		—
	warrant and reclassification to permanent equity
	Balance, end of year		$	—	$	—		$	563
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. The carrying value of long-lived assets is reviewed for impairment whenever events or changes in circumstances indicate that the assets may not be recoverable. An impairment loss is recognized 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, 2014, there have been no such impairment losses.
Preferred Stock Warrant Liability	Preferred Stock Warrant Liability
	Freestanding warrants for shares that are either putable or redeemable are classified as liabilities on the balance sheet at fair value. Therefore, the freestanding warrants that give the holders the right to purchase our convertible preferred stock are liabilities that are recorded at estimated fair value. At the end of each reporting period, changes in fair value during the period are recorded as a component of other income (expense), net.
	We adjusted the liability for changes in the estimated fair value of the warrants until the earlier of the exercise or expiration of the warrants to purchase shares of convertible preferred stock or the completion of a liquidation event, including the completion of an initial public offering, at which time the liabilities were reclassified to stockholders’ equity (deficit).
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:
	o	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.
	o	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, such that we recognize any payment that is contingent upon the achievement of a substantive milestone entirely in the period in which the milestone is achieved. A milestone is defined as an event that can only be achieved based in whole or in part on either our performance or the occurrence of a specific outcome resulting from our performance for which there is substantive uncertainty at the date the arrangement is entered into that the event will be achieved. Therefore, a milestone does not include events for which occurrence is contingent solely on the performance of a collaborative partner. To be substantive, a milestone must meet all the following criteria: the consideration receivable upon the achievement of the milestone is commensurate with either our performance after the agreement to achieve the milestone or the enhancement of value of delivered items as a result of a specific outcome resulting from our performance after the agreement to achieve the milestone, the consideration relates solely to past performance, and the consideration is reasonable relative to all of the deliverables and payment terms in the arrangement.
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 stocks and stock options. For restricted stock awards, 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 prevesting option forfeitures and record stock-based compensation expense only for those awards that are expected 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.
Net Loss Per Share of Common Stock	Net Loss Per Share of Common Stock
	We compute basic net loss per common share by dividing net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period. We did not include potentially dilutive securities consisting of options and restricted stock awards, or RSAs, to purchase common stock, preferred stock warrants, common stock warrants and convertible preferred stock in the diluted net loss per common share calculations for all periods presented because the inclusion of such shares would have had an antidilutive effect. The convertible preferred stock contained certain participation rights.
	We excluded the following securities (in thousands) from the calculation of diluted net loss per share as the effect would have been antidilutive.
			Year Ended December 31,
			2014		2013			2012
	Convertible preferred stock			—		7,209			9,824
	Options and RSAs to purchase common stock			2,388		2,338			2,347
	Warrants to purchase convertible preferred stock			—		61			87
	Warrants to purchase common stock			—		1			—
				2,388		9,609			12,258
Recently Issued Accounting Standards	Recently Issued Accounting Standards
	In May 2014, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, No. 2014-09, “Revenue from Contracts with Customers.” ASU 2014-09 supersedes the revenue recognition requirements in “Topic 605, Revenue Recognition” and requires entities to recognize revenue in a way that depicts the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 is effective retrospectively for annual or interim reporting periods beginning after December 15, 2016, with early application not permitted. We are currently evaluating the effect the adoption of ASU 2014-09 will have on our financial statements.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
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, 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		$	—
		December 31, 2013
				Basis of Fair Value Measurements
		Total		Level 1			Level 2			Level 3
	Assets
	Money market funds	$	6,456	$	6,456		$	—		$	—
	U.S. Treasury securities		18,852		18,852			—			—
	U.S. government agency securities		48,709		—			48,709			—
	Total cash equivalents and marketable securities	$	74,017	$	25,308		$	48,709		$	—
Change in Estimated Fair Value of Preferred Stock Warrant Liability on Recurring Basis	The change in the estimated fair value of the Level 3 preferred stock warrant liability on a recurring basis is summarized below (in thousands):
		Year Ended December 31,
		2014		2013			2012
	Balance, beginning of year	$	—	$	563		$	682
	Change in fair value recorded in other expense, net		—		(500	)		(119	)
	Exercises		—		(57	)		—
	Conversion of preferred stock warrant to common stock		—		(6	)		—
	warrant and reclassification to permanent equity
	Balance, end of year	$	—	$	—		$	563
Securities Excluded from Calculation of Diluted Net Loss Per Share	We excluded the following securities (in thousands) from the calculation of diluted net loss per share as the effect would have been antidilutive.
		Year Ended December 31,
		2014		2013			2012
	Convertible preferred stock		—		7,209			9,824
	Options and RSAs to purchase common stock		2,388		2,338			2,347
	Warrants to purchase convertible preferred stock		—		61			87
	Warrants to purchase common stock		—		1			—
			2,388		9,609			12,258

Cash_Equivalents_and_Marketabl1

Cash Equivalents and Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 (in thousands):
		31-Dec-14
		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
		December 31, 2013
		Amortized			Unrealized		Unrealized			Estimated
		Cost Basis			Gains		Losses			Fair Value
	Money market funds	$	6,456		$	—	$	—		$	6,456
	U.S. Treasury securities		18,848			4		—			18,852
	U.S. government agency securities		48,709			3		(3	)		48,709
			74,013			7		(3	)		74,017
	Less: cash equivalents		(6,456	)		—		—			(6,456	)
	Total marketable securities	$	67,557		$	7	$	(3	)	$	67,561
Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity	As of December 31, 2014, 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	$	129,232		$	129,239
	In one to two years		14,550			14,544
	Total marketable securities	$	143,782		$	143,783

Property_and_Equipment_Tables

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Components of Property and Equipment	Property and equipment consist of the following (in thousands):
		December 31,
		2014			2013
	Computer equipment and software	$	1,269		$	1,097
	Furniture and fixtures		731			694
	Laboratory equipment		9,978			9,596
	Leasehold improvements		2,173			2,173
			14,151			13,560
	Less: accumulated depreciation and amortization		(10,357	)		(9,816	)
	Property and equipment, net	$	3,794		$	3,744

Stockholders_Equity_Deficit_Ta

Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014		2,236,997		$	6.09
	Options granted		824,895		$	11.71
	Options exercised		(307,272	)	$	5.47
	Options forfeited		(68,182	)	$	7.7
	Options expired		(1,626	)	$	1.23
	Balance at December 31, 2014		2,684,812		$	7.85
	Options exercisable at December 31, 2014		1,409,621		$	6.13		5.76	$	29,424
	Options vested and expected to vest at December 31, 2014		2,643,893		$	7.81		7.26	$	50,727
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, 2014		1,000		$	15.64
	RSAs granted		26,500		$	12.3
	RSAs vested		(3,500	)	$	14.09
	RSAs forfeited		—		$	—
	Unvested balance at December 31, 2014		24,000		$	12.18
Schedule of Stock-Based Compensation Expenses Recognized	Total stock-based compensation expense recognized was as follows:
		Years Ended December 31
	(in thousands)	2014			2013		2012
	Research and development	$	1,761		$	968	$	705
	General and administrative		1,657			1,178		1,016
	Total	$	3,418		$	2,146	$	1,721
Schedule of Stock Option Weighted-Average Assumptions	The fair value of each award was estimated 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,
		2014			2013		2012		2014		2013		2012
	Expected term (years)	5.3-6.7			5.0-6.1		5.0-6.1		0.5		0.5			—
	Expected volatility		85.00%			85.00%		85.00%	49.0 - 83.0%			62.00%		—
	Risk-free interest rate	1.6-2.0%			0.8-2.0%		0.6-1.1%			0.10%	0.10%			—
	Expected dividend yield		0.00%			0.00%		0.00%		0.00%		0.00%		—

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
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,
		2014			2013			2012
	Federal statutory income tax rate	$	(13,098	)	$	(10,105	)	$	(9,658	)
	Nondeductible stock compensation		(501	)		455			386
	Nontaxable equity premiums		(504	)		(532	)		(452	)
	Deferred tax assets not benefitted		14,075			10,338			9,750
	Other permanent items		28			(156	)		(26	)
	Income tax expense	$	—		$	—		$	—
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,
		2014			2013
	Net operating loss carryforwards	$	73,722		$	59,552
	Research and development credit		8,685			6,564
	Reserves and accruals		7,233			5,936
	Total deferred tax assets		89,640			72,052
	Less: valuation allowance		(89,640	)		(72,052	)
	Net deferred tax assets	$	—		$	—
Schedule of Reconciliation of Unrecognized Tax Benefits	A reconciliation of our unrecognized tax benefits for the years ended December 31, 2014, 2013 and 2012, is as follows (in thousands):
		Unrecognized
		Income Tax
		Benefits
	Balance as of January 1, 2012	$	1,457
	Additions for current year tax positions		78
	Balance as of December 31, 2012		1,535
	Additions for prior 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

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
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:
	2015	$	3,108
	2016		3,363
	2017		3,461
	Total estimated minimum payments	$	9,932

Selected_Quarterly_Financial_I1

Selected Quarterly Financial Information (Tables)	12 Months Ended
	Dec. 31, 2014
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	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	)
		Quarter Ended
		March 31,			June 30,			September 30,			December 31,
	Quarterly Results of Operations	2013			2013			2013			2013
		(Unaudited)
	Revenue	$	2,975		$	3,549		$	3,482		$	3,785
	Net loss		(7,047	)		(7,274	)		(7,234	)		(7,317	)
	Basic and diluted net loss per share		(5.73	)		(5.82	)		(2.74	)		(0.43	)

Business_Additional_Informatio

Business - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
	Segment
Regulatory Assets [Abstract]
Number of operating segment	1

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Summary of Financial Instruments Measured at Fair Value (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Money market funds [Member]
Assets
Assets	$9,996	$6,456
U.S. Treasury securities [Member]
Assets
Assets	130,786	18,852
U.S. government agency securities [Member]
Assets
Assets	3,001	48,709
Total cash equivalents and marketable securities [Member]
Assets
Assets	143,783	74,017
Level 1 [Member] | Money market funds [Member]
Assets
Assets	9,996	6,456
Level 1 [Member] | U.S. Treasury securities [Member]
Assets
Assets	130,786	18,852
Level 1 [Member] | Total cash equivalents and marketable securities [Member]
Assets
Assets	140,782	25,308
Level 2 [Member] | U.S. government agency securities [Member]
Assets
Assets	3,001	48,709
Level 2 [Member] | Total cash equivalents and marketable securities [Member]
Assets
Assets	$3,001	$48,709

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 26, 2014	Feb. 28, 2005	Jun. 30, 2004
Summary Of Significant Accounting Policies [Line Items]
Warrants fair value assumptions, expected volatility rate				85.00%
Warrants fair value assumptions, expected remaining term				2 years 1 month 6 days
Warrants fair value assumptions, expected risk free interest rate				0.30%
Net number of warrants exercised	4,376				768
Gain (Loss) related the change in fair value of other income	$83,000
Reclassification of warrant liability to permanent equity	57,000		57,000
Fair value price of warrants, per share	$13
Outstanding warrant converted into a warrant to purchase, number of shares	2,304
Outstanding warrant converted into a warrant to purchase, per share	$12.30
Reclassification of warrant liability to equity	6,000		6,000
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
Series A convertible preferred stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Gain (Loss) related the change in fair value of other income	($3,000)
Outstanding warrant converted into a warrant to purchase, number of shares	2,304					81,300	2,304
Outstanding warrant converted into a warrant to purchase, per share	$12.30					$12.30	$12.30

Summary_of_Significant_Account5

Summary of Significant Accounting Policies - Change in Estimated Fair Value of Preferred Stock Warrant Liability on Recurring Basis (Detail) (USD $)	1 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Balance, beginning of year		$563	$682
Change in fair value recorded in other expense, net		-500	-119
Exercises	-57	-57
Conversion of preferred stock warrant to common stock warrant and reclassification to permanent equity	-6	-6
Balance, end of year			$563

Summary_of_Significant_Account6

Summary of Significant Accounting Policies - Securities Excluded from Calculation of Diluted Net Loss Per Share (Detail)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net (loss) income per share	2,388	9,609	12,258
Convertible preferred stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net (loss) income per share		7,209	9,824
Options and RSAs to purchase common stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net (loss) income per share	2,388	2,338	2,347
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 (loss) income per share		61	87
Warrants to purchase common stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net (loss) income per share		1

Cash_Equivalents_and_Marketabl2

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	$133,786	$67,557
Unrealized Gains	18	7
Unrealized Losses	-17	-3
Marketable securities	133,787	67,561
Money market funds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	9,996	6,456
Marketable securities	9,996	6,456
U.S. Treasury securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	130,785	18,848
Unrealized Gains	18	4
Unrealized Losses	-17
Marketable securities	130,786	18,852
Marketable securities including cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	143,782	74,013
Unrealized Gains	18	7
Unrealized Losses	-17	-3
Marketable securities	143,783	74,017
U.S. government agency securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	3,001	48,709
Unrealized Gains		3
Unrealized Losses		-3
Marketable securities	3,001	48,709
Cash equivalents [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost Basis	9,996	6,456
Marketable securities	$9,996	$6,456

Cash_Equivalents_and_Marketabl3

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Amortized Cost And Fair Value Debt Securities [Abstract]
Amortized Cost, In one year or less	$129,232
Amortized Cost, In one to two years	14,550
Total marketable securities, Amortized Cost	143,782
Estimated Fair Value, In one year or less	129,239
Estimated Fair Value, In one to two years	14,544
Total marketable securities, Estimated Fair Value	$143,783

Cash_Equivalents_and_Marketabl4

Cash Equivalents and Marketable Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Gross unrealized losses on marketable securities	$17,000
Sales of available-for-sale securities	$0

Property_and_Equipment_Compone

Property and Equipment - Components of Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant And Equipment [Line Items]
Property and equipment, gross	$14,151	$13,560
Less: accumulated depreciation and amortization	-10,357	-9,816
Property and equipment, net	3,794	3,744
Computer equipment and software [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	1,269	1,097
Furniture and fixtures [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	731	694
Laboratory equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	9,978	9,596
Leasehold improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, gross	$2,173	$2,173

Preferred_Stock_and_Common_Sto1

Preferred Stock and Common Stock Warrant - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2013	Jan. 26, 2014	Dec. 31, 2007	Feb. 28, 2005	Jun. 30, 2004
				Warrant
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares	2,304
Warrant to purchase preferred stock, per share	$12.30
Gain (Loss) related the change in fair value of other income	$83
Reclassification of warrant liability to equity	6	6
Net number of warrants exercised	4,376		768
Number of warrants				2
Reclassification of warrant liability to permanent equity	57	57
Fair value price of warrants, per share	$13
Warrants one [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares				44,715
Warrants two [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares				36,585
Series A convertible preferred stock [Member]
Class Of Warrant Or Right [Line Items]
Warrant to purchase preferred stock, number of shares	2,304				81,300	2,304
Warrant to purchase preferred stock, per share	$12.30				$12.30	$12.30
Gain (Loss) related the change in fair value of other income	($3)

Convertible_Preferred_Stock_Ad

Convertible Preferred Stock - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
Temporary Equity Disclosure [Abstract]
Preferred stock converted into common stock	9,929,159

Stockholders_Equity_Deficit_Ad

Stockholders' Equity (Deficit) - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Sep. 23, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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		$8.39	$5.05	$4.06
Total intrinsic value of options exercised		$4,200,000	$1,100,000	$300,000
Total unrecognized compensation expense		8,000,000
Unrecognized compensation expense expected to recognize, weighted-average period		3 years
Stock-based compensation expense		3,418,000	2,146,000	1,721,000
Expected dividend yield		0.00%
Employees and directors [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		2,900,000	2,100,000	1,700,000
Non employee options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense		134,000	79,000	66,000
Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total unrecognized compensation expense		300,000
Unrecognized compensation expense expected to recognize, weighted-average period		2 years 6 months
Total fair value of share vesting		54,000	0
Restricted Stock Awards [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period		2 years
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		3,358,568
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		334,561
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, 2014, 334,561 shares of common stock were available for issuance under the ESPP.
Total unrecognized compensation expense		100,000
Unrecognized compensation expense expected to recognize, weighted-average period		4 years 6 months
Stock-based compensation expense		$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		83,860	0

Stockholders_Equity_Deficit_Sc

Stockholders' Equity (Deficit) - Schedule of Option Activity under Stock Plans and Related Information (Detail) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	2,236,997
Number of Shares, Options granted	824,895
Number of Shares, Options exercised	-307,272
Number of Shares, Options forfeited	-68,182
Number of Shares, Options expired	-1,626
Number of Shares, Ending balance	2,684,812
Number of Shares, Options exercisable	1,409,621
Number of Shares, Options vested and expected to vest	2,643,893
Weighted-Average Exercise Price Per Share, Options beginning balance	$6.09
Weighted-Average Exercise Price Per Share, Options granted	$11.71
Weighted-Average Exercise Price Per Share, Options exercised	$5.47
Weighted-Average Exercise Price Per Share, Options forfeited	$7.70
Weighted-Average Exercise Price Per Share, Options expired	$1.23
Weighted-Average Exercise Price Per Share, Options ending balance	$7.85
Weighted-Average Exercise Price Per Share, Options exercisable	$6.13
Weighted-Average Exercise Price Per Share, Options vested and expected to vest	$7.81
Weighted-Average Remaining Contractual Terms, Options exercisable	5 years 9 months 4 days
Weighted-Average Remaining Contractual Terms, Options vested and expected to vest	7 years 3 months 4 days
Aggregate Intrinsic Value, Options exercisable	$29,424
Aggregate Intrinsic Value, Options vested and expected to vest	$50,727

Stockholders_Equity_Deficit_Sc1

Stockholders' Equity (Deficit) - Schedule of Unvested Restricted Stock Awards (Detail) (Restricted Stock Awards [Member], USD $)	12 Months Ended
	Dec. 31, 2014
Restricted Stock Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Unvested, Beginning balance	1,000
Granted, Number of Shares	26,500
Vested, Number of Shares	-3,500
Number of Shares, Unvested, Ending balance	24,000
Number of Shares, Unvested, Beginning balance	$15.64
Granted, Weighted Average Grant Date Fair Value	$12.30
Vested, Weighted Average Grant Date Fair Value	$14.09
Number of Shares, Unvested, Ending balance	$12.18

Stockholders_Equity_Deficit_Sc2

Stockholders' Equity (Deficit) - Schedule of Stock-Based Compensation Expenses Recognized (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$3,418	$2,146	$1,721
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	1,761	968	705
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$1,657	$1,178	$1,016

Stockholders_Equity_Deficit_Sc3

Stockholders' Equity (Deficit) - Schedule of Stock Option Weighted-Average Assumptions (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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%	85.00%
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 3 months 18 days	5 years	5 years
Risk-free interest rate	1.60%	0.80%	0.60%
Options [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 years 8 months 12 days	6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate	2.00%	2.00%	1.10%
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (years)	6 months	6 months	0 years
Expected volatility		62.00%
Risk-free interest rate	0.10%	0.10%
Expected dividend yield	0.00%	0.00%
ESPP [Member] | Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	49.00%
ESPP [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	83.00%

Employee_Benefit_Plans_Additio

Employee Benefit Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
Eligible employees contribution percentage	100.00%

Collaborative_Research_and_Dev1

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

1 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

1 Months Ended

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2014

Oct. 31, 2013

Sep. 30, 2013

Aug. 31, 2010

Dec. 31, 2011

Apr. 30, 2012

Apr. 30, 2014

31-May-11

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment

$6,000,000

Deferred revenue recognition research period

5 years

Research funding

2,000,000

Convertible preferred stock issued

0

0

Proceeds from issuance of preferred stock

6,819,000

Recognized revenue under collaboration arrangement

3,200,000

2,200,000

Research funding payment received

1,000,000

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

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment

7,000,000

Deferred revenue recognition research period

3 years

Research funding

6,300,000

Research funding received

0

1,500,000

4,200,000

Deferred revenue recognized under collaboration arrangement

1,900,000

Milestone payments received

300,000

300,000

Extension fee

200,000

Extended evaluation period

8 months

Proceeds from options exercised

1,500,000

Recognized revenue under collaboration arrangement

3,400,000

5,800,000

5,800,000

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

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] | Respiratory Diseases Discovery Collaboration [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Upfront payment

7,500,000

Deferred revenue recognition research period

4 years

Research funding

10,500,000

Research funding received

3,900,000

3,400,000

1,300,000

Option and selection payments

1,800,000

Deferred revenue recognized under collaboration arrangement

4,300,000

6,800,000

Recognized revenue under collaboration arrangement

6,400,000

5,600,000

3,200,000

Research funding payment received

1,000,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Phase 1 clinical trial [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Potential milestone payments

6,500,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Phase 2 clinical trial [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] | Respiratory Diseases Discovery Collaboration [Member] | Track 2 Target [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Eligible potential milestone payment

23,000,000

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] | Respiratory Diseases Discovery Collaboration [Member] | Development costs [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Collaboration receivable

300,000

100,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Each Additional Screening Assays [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Maximum additional research funding to be received

1,000,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Two Additional Screening Assays [Member]

Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]

Maximum additional research funding to be received

2,000,000

Glaxo Smith Kline LLC [Member] | Respiratory Diseases Discovery Collaboration [Member] | Series A3 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

Collaborative_Research_and_Dev2

Collaborative Research and Development Agreements - Additional Information 1 (Detail) (USD $)	1 Months Ended	12 Months Ended		24 Months Ended	1 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment	$6
Recognized revenue under collaboration arrangement		3.2	2.2
Expected evaluation period to complete initial research activities	2 years
Technology access fee received		2.2	2.2	4.4
Technology access fee	6.6
Contingent technology access fee	2.2
Research funding	2
Research funding payment received		1
Deferred revenue recognition research period	5 years
Deferred revenue relating to collaboration agreement		6.5	6.2	6.5
Maximum [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Eligible cash receipt for target evaluation and selection		0.4
FP-1039 License and Collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment					50
Recognized revenue under collaboration arrangement		0.1	0.1			0.9
Collaboration arrangement, contingent payments		435		435
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
UCB Pharma S.A. [Member] | Development costs [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration receivable		$0.10	$0.20

Collaborative_Research_and_Dev3

Collaborative Research and Development Agreements - Additional Information 2 (Detail) (USD $)	1 Months Ended	12 Months Ended		1 Months Ended
	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Apr. 30, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment	$6,000,000
Deferred revenue recognition research period	5 years
Research funding payment received		1,000,000
Aggregate purchase price		58,728,000	63,849,000
Recognized revenue under collaboration arrangement		3,200,000	2,200,000
Deferred revenue relating to collaboration agreement		6,500,000	6,200,000
Bristol-Myers Squibb Company [Member] | Immuno-oncology collaboration [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment				20,000,000
Research funding received		9,500,000
Research agreement 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
Deferred revenue recognition research period		5 years
Research funding payment received		3,400,000
Issuance of stock, Shares		994,352
Price per share		$21.16
Aggregate purchase price		21,000,000
Deferred revenue recognized under collaboration arrangement		2,400,000
Recognized revenue under collaboration arrangement		6,000,000
Deferred revenue relating to collaboration agreement		$19,700,000

Collaborative_Research_and_Dev4

Collaborative Research and Development Agreements - Additional Information 3 (Detail) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement termination notice period	90 days
Research agreement term description	Either party may terminate the agreement with written notice (i) if the other party is in material breach and such breach has not been cured within the applicable cure period, (ii) if either party deems it necessary to protect the safety, health or welfare of the subjects enrolled in a clinical trial or (iii) 90 days following the commencement of a clinical hold.
Bristol-Myers Squibb Company [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
One-time agreement fee, receivable	30
Collaboration Fee, Payable	30
Percentage Of Collaboration Fee Payable On Aggregate Purchase Price Paid To Stockholders	10.00%

Acquired_Technologies_Addition

Acquired Technologies - Additional Information (Detail) (Galaxy Biotech, LLC [Member], USD $)	1 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2014
	Installment
Galaxy Biotech, LLC [Member]
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront license payment	$3
Number of installments	2
Milestone payment		$2.60

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Line Items]
Income tax expense	$0	$0	$0
Increase in valuation allowance	17,600,000	12,100,000
Accrued interest or penalties	0	0
Incurred interest and penalties	0	0
Significant increase (decrease) to uncertain tax positions	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]
Operating loss carryforwards	179,700,000
Tax credits available to offset future tax	6,900,000
Operating loss carryforwards, Expiration date	31-Dec-24
Tax credit carryforwards, Expiration date	31-Dec-23
State tax authority [Member]
Income Tax [Line Items]
Operating loss carryforwards	205,900,000
Operating loss carryforwards, Expiration date	31-Dec-15
Research and development tax credits [Member] | State tax authority [Member]
Income Tax [Line Items]
Tax credits available to offset future tax	$5,600,000

Income_Taxes_Schedule_of_Recon

Income Taxes - Schedule of Reconciliation of Federal Statutory Income Tax Rate (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Abstract]
Federal statutory income tax rate	($13,098)	($10,105)	($9,658)
Nondeductible stock compensation	-501	455	386
Nontaxable equity premiums	-504	-532	-452
Other permanent items	28	-156	-26
Income tax expense	$0	$0	$0

Income_Taxes_Schedule_of_Defer

Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred Tax Assets Liabilities Net [Abstract]
Net operating loss carryforwards	$73,722	$59,552
Research and development credit	8,685	6,564
Reserves and accruals	7,233	5,936
Total deferred tax assets	89,640	72,052
Less: valuation allowance	-89,640	-72,052
Net deferred tax assets	$0	$0

Income_Taxes_Schedule_of_Recon1

Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
[ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward]
Beginning Balance	$1,781	$1,535	$1,457
Additions for prior year tax positions	11	27
Additions for current year tax positions	445	219	78
Ending Balance	$2,237	$1,781	$1,535

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2010	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Leases Operating [Abstract]
Lease option additional extend term		3 years
Lease expiration		31-Dec-17
Allowance for cost of leasehold improvements	$0.20
Deferred rent		2.1	2.7
Incentive to lessee		1.7
Unamortized leasehold improvement incentive total		0.7	0.9
Unamortized leasehold improvement incentive included in other long-term liabilities		0.5	0.7
Rent expense		$1.90	$1.90	$1.90

Commitments_and_Contingencies_2

Commitments and Contingencies - Schedule of Estimated Future Minimum Commitments for Operating Leases (Detail) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Leases Operating [Abstract]
2015	$3,108
2016	3,363
2017	3,461
Total estimated minimum payments	$9,932

Selected_Quarterly_Financial_I2

Selected Quarterly Financial Information - Summary of Quarterly Financial Information (Detail) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$4,645	$6,059	$4,981	$3,546	$3,785	$3,482	$3,549	$2,975
Net loss	($11,826)	($7,088)	($9,866)	($8,644)	($7,317)	($7,234)	($7,274)	($7,047)	($37,424)	($28,872)	($27,595)
Basic and diluted net loss per share	($0.55)	($0.33)	($0.46)	($0.46)	($0.43)	($2.74)	($5.82)	($5.73)

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2015
Subsequent Event [Line Items]
Proceeds from issuances of common stock, net of issuance costs	$58,728	$63,849
Common Stock [Member]
Subsequent Event [Line Items]
Issuance of stock, Shares	3,450,000
Subsequent Events [Member] | Follow On Public Offering [Member]
Subsequent Event [Line Items]
Proceeds from issuances of common stock, net of issuance costs			$78,700
Subsequent Events [Member] | Follow On Public Offering [Member] | Common Stock [Member]
Subsequent Event [Line Items]
Issuance of stock, Shares			3,829,994