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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2014	Aug. 05, 2014
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	Q2
Trading Symbol	FPRX
Entity Registrant Name	FIVE PRIME THERAPEUTICS INC
Entity Central Index Key	0001175505
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		21,520,554

Condensed Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 13,059	$ 8,161
Marketable securities	127,522	67,561
Receivable from collaborative partners	177	296
Prepaid and other current assets	1,257	1,640
Total current assets	142,015	77,658
Property and equipment, net	3,957	3,744
Other long-term assets	442	389
Total assets	146,414	81,791
Liabilities and stockholders' equity
Accounts payable	767	348
Accrued personnel-related expenses	2,319	2,957
Other accrued liabilities	4,834	2,056
Deferred revenue, current portion	11,647	7,913
Deferred rent, current portion	590	549
Total current liabilities	20,157	13,823
Deferred revenue, long-term portion	22,961	7,123
Deferred rent, long-term portion	1,830	2,146
Other long-term liabilities	560	673
Commitments
Common stock	21	17
Preferred stock
Additional paid-in capital	270,928	209,580
Accumulated other comprehensive income	41	3
Accumulated deficit	(170,084)	(151,574)
Total stockholders' equity	100,906	58,026
Total liabilities and stockholders' equity	$ 146,414	$ 81,791

Condensed Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Collaboration revenue	$ 4,981	$ 3,549	$ 8,527	$ 6,524
Operating expenses:
Research and development	11,873	8,585	20,799	16,515
General and administrative	3,024	2,386	6,304	4,778
Total operating expenses	14,897	10,971	27,103	21,293
Loss from operations	(9,916)	(7,422)	(18,576)	(14,769)
Interest income	55	13	91	28
Other income (expense), net	(5)	135	(25)	420
Net loss	$ (9,866)	$ (7,274)	$ (18,510)	$ (14,321)
Basic and diluted net loss per common share	$ (0.46)	$ (5.82)	$ (0.92)	$ (11.55)
Shares used to compute basic and diluted net loss per common share	21,465	1,250	20,160	1,240

Condensed Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (9,866)	$ (7,274)	$ (18,510)	$ (14,321)
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities	22	(4)	38	(6)
Comprehensive loss	$ (9,844)	$ (7,278)	$ (18,472)	$ (14,327)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Condensed Statement of Cash Flows - USD ($) $ in Thousands	6 Months Ended
Condensed Statement of Cash Flows - USD ($) $ in Thousands	Jun. 30, 2014	Jun. 30, 2013
Operating activities
Net loss	$ (18,510)	$ (14,321)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	772	855
Stock-based compensation expense	1,376	1,035
Amortization of premium on marketable securities	628	209
Revaluation of preferred stock warrant liability		(420)
Changes in operating assets and liabilities:
Receivable from collaborative partners	119	255
Prepaid, other current assets, and other long-term assets	(33)	(157)
Accounts payable	419	(213)
Accrued personnel-related expenses	(638)	(575)
Deferred revenue	19,660	4,228
Deferred rent	(275)	126
Other accrued liabilities and other long-term liabilities	2,940	364
Net cash provided by (used in) operating activities	6,458	(8,614)
Investing activities
Purchases of marketable securities	(106,451)	(11,577)
Maturities of marketable securities	45,900	18,685
Purchases of property and equipment	(985)	(524)
Net cash (used in) provided by investing activities	(61,536)	6,584
Financing activities
Proceeds from public offering of common stock, net	40,099
Proceeds from the sale of common stock to collaborative partner	18,629
Payments of financing costs for an initial public offering		(546)
Proceeds from issuance of common stock under equity incentive plans	1,248	88
Payments under capital lease obligation		(8)
Net cash provided by (used in) financing activities	59,976	(466)
Net increase (decrease) in cash and cash equivalents	4,898	(2,496)
Cash and cash equivalents at beginning of period	8,161	11,391
Cash and cash equivalents at end of period	$ 13,059	8,895
Supplemental schedule of noncash financing activities
Accrued and deferred offering costs		$ 967

Description of Business

6 Months Ended

Jun. 30, 2014

Accounting Policies [Abstract]

Description of Business

1.	Description of 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.

Unaudited Interim Financial Information

The accompanying financial information as of June 30, 2014 is unaudited. The Condensed Financial Statements included in this report reflect all adjustments (consisting only of normal recurring adjustments) that our management considers necessary for the fair statement of the results of operations for the interim periods covered and of the financial condition of the Company at the date of the interim balance sheet. The December 31, 2013 Condensed Balance Sheet was derived from audited financial statements, but does not include all disclosures required by generally accepted accounting principles in the United States of America, or GAAP. The results for interim periods are not necessarily indicative of the results for the entire year or any other interim period. The accompanying Condensed Financial Statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2013 filed with the U.S. Securities and Exchange Commission.

Follow-on Public Offering

In February 2014, we completed a public offering of 3,450,000 shares of our common stock, or our Follow-on Public Offering, which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option. We received net proceeds of $40.1 million, after underwriting discounts, commissions and offering expenses, from the Follow-on Public Offering.

Summary of Significant Accounting Policies

6 Months Ended

Jun. 30, 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 GAAP requires management to make estimates and assumptions about future events that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements as well as reported amounts of revenue and expenses during the reporting period. Actual results could differ materially from those estimates.

Reclassifications

We have reclassified certain prior period amounts to conform to the current period presentation. We reclassified certain liabilities, primarily those related to unbilled receipts, from accounts payable to other accrued liabilities on the balance sheets and made related conforming reclassifications on the statement of cash flows.

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; 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, were derived from observable market data, or, if not directly observable, were derived from or corroborated by other observable market data.

In certain cases where there is limited activity or less transparency around inputs to valuation, we classify securities as Level 3 within the valuation hierarchy. As of June 30, 2013, our Level 3 liability consists of a preferred stock warrant liability that we measured at estimated fair value.

The following table summarizes, for assets and the liability recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):


	JUNE 30, 2014
			BASIS OF FAIR VALUE MEASUREMENTS
	TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
Assets
Money market funds	$	8,651	$	8,651	$	—	$	—
U.S. Treasury securities		102,630		102,630		—		—
U.S. government agency securities		24,892		—		24,892		—

Total cash equivalents and marketable securities	$	136,173	$	111,281	$	24,892	$	—


	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	$	—

We estimated the fair value of the preferred stock warrant liability using the Black-Scholes option-pricing model, or OPM. 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 June 30, 2013, as more certainty developed regarding a possible initial public offering, we began transitioning the valuation methodology for the preferred stock warrant liability for those warrants that would expire upon the initial public offering from the OPM to a probability-weighted expected return method, or PWERM. We continued to value the warrants that would not expire upon the initial public offering using the OPM. The PWERM is a scenario-based analysis that estimates the value of each class of equity, including the preferred stock warrants, on the probability-weighted present value of expected future investment returns, considering each of the possible outcomes available to us, as well as the rights of each equity class. The PWERM estimates the value of each equity class under each of three possible future scenarios—initial public offering, sale and remain a private company. The value under each scenario is then probability weighted and the resulting weighted values summed to determine the fair value per share of each equity class. In connection with the completion of our initial public offering in September 2013, or our IPO, substantially all of the warrants were automatically net exercised for a total of 4,376 shares of common stock, pursuant to the terms of the warrants and 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, automatically net exercised for a total of 768 shares of common stock on January 26, 2014.

The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):


	SIX MONTHS ENDED JUNE 30, 2013
Balance, beginning	$	563
Change in fair value recorded in other income, net		(420	)

Balance, ending	$	143

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 stock options, 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 contains certain participation rights.

We excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Convertible preferred stock		—		9,929		—		9,929
Options to purchase common stock		2,141		2,216		2,177		2,426
Warrants to purchase convertible preferred stock		—		84		—		84

		2,141		12,229		2,177		12,439

Deferred Offering Costs

We capitalize deferred offering costs, which primarily consist of direct incremental legal and accounting fees relating to our IPO. We offset the deferred offering costs against our initial public offering proceeds upon the consummation of our IPO in September 2013. As of June 30, 2013, $1.5 million of deferred offering costs were capitalized in other long-term assets on our balance sheets.

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 Marketable Securities

6 Months Ended

Jun. 30, 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 (in thousands):


	JUNE 30, 2014
	AMORTIZED COST BASIS			UNREALIZED GAINS		UNREALIZED LOSSES			ESTIMATED FAIR VALUE
	*(unaudited)*
Money market funds	$	8,651		$	—	$	—		$	8,651
U.S. Treasury securities		102,595			38		(3	)		102,630
U.S. government agency securities		24,886			6		—			24,892

		136,132			44		(3	)		136,173
Less: cash equivalents		(8,651	)		—		—			(8,651	)

Total marketable securities	$	127,481		$	44	$	(3	)	$	127,522


	December 31, 2013
	AMORTIZED COST BASIS			UNREALIZED GAINS		UNREALIZED LOSSES			ESTIMATED 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 June 30, 2014, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):


	Amortized Cost		Estimated Fair Value
Debt securities maturing:
In one year or less	$	102,079	$	102,104
In one to two years		34,053		34,069

Total marketable securities	$	136,132	$	136,173

We determined that the gross unrealized losses of $3,000 on our marketable securities as of June 30, 2014 were temporary in nature. 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 June 30, 2014. There were no sales of available-for-sale securities in any of the periods presented.

Equity Incentive Plans

6 Months Ended

Jun. 30, 2014

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity Incentive Plans

4.	Equity Incentive Plans

The following table summarizes option activity under our stock plans and related information:


	OPTIONS OUTSTANDING
	NUMBER OF SHARES			WEIGHTED AVERAGE EXERCISE PRICE PER SHARE
Balance at December 31, 2013		2,236,997		$	6.09
Options granted		139,820		$	13.24
Options exercised		(159,161	)	$	5.23
Options forfeited		(19,931	)	$	7.19

Balance at June 30, 2014		2,197,725		$	6.60

Options exercisable		1,344,411		$	5.80

As of June 30, 2014, there were 4,013,766 shares of common stock available for future issuance under our 2013 Omnibus Incentive Plan.

As of June 30, 2014, options to purchase 2,174,205 shares of common stock were outstanding, which are fully vested or expected to vest with a weighted-average exercise price of $6.58 per share and a weighted-average remaining contractual term of 7.1 years. As of June 30, 2014, the weighted-average remaining contractual term for options exercisable was 6.2 years.

Stock-Based Compensation

We calculate employee stock-based compensation expense based on awards ultimately expected to vest reduced by estimated forfeitures. We estimate forfeitures at the time of grant and revise forfeitures, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Total stock-based compensation expense recognized was as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Research and development	$	369	$	188	$	729	$	400
General and administrative		371		223		647		635

Total	$	740	$	411	$	1,376	$	1,035

In February 2013, we amended stock options held by our former CEO to extend the post-termination exercise period for the former CEO’s outstanding vested options from 18 months to 20 months, which resulted in additional incremental stock-based compensation of $157,000 in the first quarter of 2013.

We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following assumptions:


	Three Months Ended June 30,						Six Months Ended June 30,
	2014			2013			2014			2013
Expected term (years)		5.3-6.1			6.0			5.3-6.1			5.0-6.1
Expected volatility		85	%		85	%		85	%		85	%
Risk-free interest rate		1.6-1.9	%		1.2	%		1.6-1.9	%		0.8-1.2	%
Expected dividend yield		0	%		0	%		0	%		0	%

As of June 30, 2014, we had $4.5 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 2.3 years.

Collaborative Research and Development Agreements

6 Months Ended

Jun. 30, 2014

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaborative Research and Development Agreements

5.	Collaborative Research and Development Agreements

Bristol-Myers Squibb Company

On March 14, 2014, we entered into a research collaboration and license agreement, or immuno-oncology 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 collaboration. We received an upfront payment of $20.0 million from BMS in April 2014 in connection with our entry into the immuno-oncology 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 Financial Accounting Standards Board 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 collaboration as a single unit of accounting and recognize the immuno-oncology 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 $1.3 million of research funding during the six months ended June 30, 2014 related to research we performed under the immuno-oncology collaboration.

We are eligible to receive certain contingent payments with respect to each target subject to the immuno-oncology 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 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 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 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, which we will recognize in the same manner as the $20.0 million up-front payment.

During the three and six months ended June 30, 2014, we recognized $1.9 million and $2.1 million, respectively, of revenue under the immuno-oncology collaboration. As of June 30, 2014, we had deferred revenue relating to the immuno-oncology collaboration of $21.5 million.

The immuno-oncology collaboration will terminate upon the expiration of all payment obligations under the collaboration. In addition, BMS may terminate the immuno-oncology 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 party fails to cure the breach or immediately upon certain insolvency events.

GlaxoSmithKline

Respiratory Diseases Discovery Collaboration

In April 2014, we amended our research collaboration and license agreement, referred to as the respiratory diseases collaboration, with Glaxo Group Limited, or GSK UK, that we originally entered into in April 2012 to identify new therapeutic approaches to treat refractory asthma and chronic obstructive pulmonary disease function. Pursuant to the respiratory diseases collaboration, GSK UK has an option to elect to include additional screening assays under the research plan. The amendment allows GSK UK to terminate any additional screening assay it elects under the research plan within six months of so electing. Concurrent with the amendment, GSK UK exercised its option and expanded the research plan to include two additional screening assays. In connection with GSK UK’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 in the event that GSK UK does not terminate either additional screening assay by October 2014. The $2.0 million in additional research funding is amortized over the remaining research period.

Description of Business (Policies)

6 Months Ended

Jun. 30, 2014

Accounting Policies [Abstract]

Unaudited Interim Financial Information

Unaudited Interim Financial Information

Use of Estimates

Use of Estimates

Reclassifications

Reclassifications

Fair Value of Financial Instruments

Fair Value of Financial Instruments

Level 1—Quoted prices in active markets for identical assets or liabilities;

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.


	JUNE 30, 2014
			BASIS OF FAIR VALUE MEASUREMENTS
	TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
Assets
Money market funds	$	8,651	$	8,651	$	—	$	—
U.S. Treasury securities		102,630		102,630		—		—
U.S. government agency securities		24,892		—		24,892		—

Total cash equivalents and marketable securities	$	136,173	$	111,281	$	24,892	$	—


	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	$	—

The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):


	SIX MONTHS ENDED JUNE 30, 2013
Balance, beginning	$	563
Change in fair value recorded in other income, net		(420	)

Balance, ending	$	143

Net Loss Per Share of Common Stock

Net Loss Per Share of Common Stock

We excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Convertible preferred stock		—		9,929		—		9,929
Options to purchase common stock		2,141		2,216		2,177		2,426
Warrants to purchase convertible preferred stock		—		84		—		84

		2,141		12,229		2,177		12,439

Deferred Offering Costs

Deferred Offering Costs

Recently Issued Accounting Standards

Recently Issued Accounting Standards

Summary of Significant Accounting Policies (Tables)

6 Months Ended

Jun. 30, 2014

Accounting Policies [Abstract]

Summary of Assets and Liabilities Recorded at Fair Value


	JUNE 30, 2014
			BASIS OF FAIR VALUE MEASUREMENTS
	TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
Assets
Money market funds	$	8,651	$	8,651	$	—	$	—
U.S. Treasury securities		102,630		102,630		—		—
U.S. government agency securities		24,892		—		24,892		—

Total cash equivalents and marketable securities	$	136,173	$	111,281	$	24,892	$	—


	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

The change in the estimated fair value of the preferred stock warrant liability is summarized below (in thousands):


	SIX MONTHS ENDED JUNE 30, 2013
Balance, beginning	$	563
Change in fair value recorded in other income, net		(420	)

Balance, ending	$	143

Securities Excluded from Calculation of Diluted Net Loss per Share

We excluded the following securities from the calculation of diluted net loss per share as the effect would have been antidilutive (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Convertible preferred stock		—		9,929		—		9,929
Options to purchase common stock		2,141		2,216		2,177		2,426
Warrants to purchase convertible preferred stock		—		84		—		84

		2,141		12,229		2,177		12,439

Cash Equivalents and Marketable Securities (Tables)

6 Months Ended

Jun. 30, 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 (in thousands):


	JUNE 30, 2014
	AMORTIZED COST BASIS			UNREALIZED GAINS		UNREALIZED LOSSES			ESTIMATED FAIR VALUE
	*(unaudited)*
Money market funds	$	8,651		$	—	$	—		$	8,651
U.S. Treasury securities		102,595			38		(3	)		102,630
U.S. government agency securities		24,886			6		—			24,892

		136,132			44		(3	)		136,173
Less: cash equivalents		(8,651	)		—		—			(8,651	)

Total marketable securities	$	127,481		$	44	$	(3	)	$	127,522


	December 31, 2013
	AMORTIZED COST BASIS			UNREALIZED GAINS		UNREALIZED LOSSES			ESTIMATED 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 June 30, 2014, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):


	Amortized Cost		Estimated Fair Value
Debt securities maturing:
In one year or less	$	102,079	$	102,104
In one to two years		34,053		34,069

Total marketable securities	$	136,132	$	136,173

Equity Incentive Plans (Tables)

6 Months Ended

Jun. 30, 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
	NUMBER OF SHARES			WEIGHTED AVERAGE EXERCISE PRICE PER SHARE
Balance at December 31, 2013		2,236,997		$	6.09
Options granted		139,820		$	13.24
Options exercised		(159,161	)	$	5.23
Options forfeited		(19,931	)	$	7.19

Balance at June 30, 2014		2,197,725		$	6.60

Options exercisable		1,344,411		$	5.80

Schedule of Stock-Based Compensation Expenses Recognized

Total stock-based compensation expense recognized was as follows (in thousands):


	Three Months Ended June 30,				Six Months Ended June 30,
	2014		2013		2014		2013
Research and development	$	369	$	188	$	729	$	400
General and administrative		371		223		647		635

Total	$	740	$	411	$	1,376	$	1,035

Schedule of Stock Option Assumptions

We estimated the fair value of each stock option using the Black-Scholes option-pricing model based on the date of grant of such stock option with the following assumptions:


	Three Months Ended June 30,						Six Months Ended June 30,
	2014			2013			2014			2013
Expected term (years)		5.3-6.1			6.0			5.3-6.1			5.0-6.1
Expected volatility		85	%		85	%		85	%		85	%
Risk-free interest rate		1.6-1.9	%		1.2	%		1.6-1.9	%		0.8-1.2	%
Expected dividend yield		0	%		0	%		0	%		0	%

Description of Business - Additional Information (Detail) $ in Thousands	1 Months Ended	6 Months Ended
	Feb. 28, 2014 shares	Jun. 30, 2014 USD ($) Segment
Regulatory Assets [Abstract]
Number of operating segment \| Segment		1
Number of common stock issued through Public Offering \| shares	3,450,000
Net proceeds after underwriting discounts, commissions and offering expenses \| $		$ 40,099

Summary of Significant Accounting Policies - Summary of Assets and Liabilities Recorded at Fair Value (Detail) - USD ($) $ in Thousands	Jun. 30, 2014	Dec. 31, 2013
Money market funds [Member]
Assets
Assets	$ 8,651	$ 6,456
U.S. Treasury securities [Member]
Assets
Assets	102,630	18,852
U.S. government agency securities [Member]
Assets
Assets	24,892	48,709
Total cash equivalents and marketable securities [Member]
Assets
Assets	136,173	74,017
Level 1 [Member] \| Money market funds [Member]
Assets
Assets	8,651	6,456
Level 1 [Member] \| U.S. Treasury securities [Member]
Assets
Assets	102,630	18,852
Level 1 [Member] \| U.S. government agency securities [Member]
Assets
Assets
Level 1 [Member] \| Total cash equivalents and marketable securities [Member]
Assets
Assets	111,281	25,308
Level 2 [Member] \| Money market funds [Member]
Assets
Assets
Level 2 [Member] \| U.S. Treasury securities [Member]
Assets
Assets
Level 2 [Member] \| U.S. government agency securities [Member]
Assets
Assets	24,892	48,709
Level 2 [Member] \| Total cash equivalents and marketable securities [Member]
Assets
Assets	24,892	$ 48,709
Level 3 [Member] \| Money market funds [Member]
Assets
Assets
Level 3 [Member] \| U.S. Treasury securities [Member]
Assets
Assets
Level 3 [Member] \| U.S. government agency securities [Member]
Assets
Assets
Level 3 [Member] \| Total cash equivalents and marketable securities [Member]
Assets
Assets

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Sep. 30, 2013	Jan. 26, 2014	Jun. 30, 2013
Equity [Abstract]
Net number of warrants exercised	4,376
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
Net number of warrants exercised		768
Deferred offering costs capitalized in other long-term assets			$ 1.5

Summary of Significant Accounting Policies - Change in Estimated Fair Value of Preferred Stock Warrant Liability (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2013 USD ($)
Fair Value Disclosures [Abstract]
Balance, beginning of year	$ 563
Change in fair value recorded in other income, net	(420)
Balance, end of year	$ 143

Summary of Significant Accounting Policies - Securities Excluded from Calculation of Diluted Net Loss per Share (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net loss per share	2,141	12,229	2,177	12,439
Convertible preferred stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net loss per share		9,929		9,929
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net loss per share	2,141	2,216	2,177	2,426
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 per share		84		84

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	$ 127,481	$ 67,557
UNREALIZED GAINS	44	7
UNREALIZED LOSSES	(3)	(3)
ESTIMATED FAIR VALUE	127,522	67,561
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	8,651	6,456
UNREALIZED GAINS
UNREALIZED LOSSES
ESTIMATED FAIR VALUE	8,651	6,456
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	102,595	18,848
UNREALIZED GAINS	38	4
UNREALIZED LOSSES	(3)
ESTIMATED FAIR VALUE	102,630	18,852
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	24,886	48,709
UNREALIZED GAINS	6	3
UNREALIZED LOSSES		(3)
ESTIMATED FAIR VALUE	24,892	48,709
Marketable securities including cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	136,132	74,013
UNREALIZED GAINS	44	7
UNREALIZED LOSSES	(3)	(3)
ESTIMATED FAIR VALUE	136,173	74,017
Cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	(8,651)	(6,456)
UNREALIZED GAINS
UNREALIZED LOSSES
ESTIMATED FAIR VALUE	$ (8,651)	$ (6,456)

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) $ in Thousands	Jun. 30, 2014 USD ($)
Debt securities maturing:
Amortized Cost, In one year or less	$ 102,079
Amortized Cost, In one to two years	34,053
Total marketable securities, Amortized Cost	136,132
Estimated Fair Value, In one year or less	102,104
Estimated Fair Value, In one to two years	34,069
Total marketable securities, Estimated Fair Value	$ 136,173

Cash Equivalents and Marketable Securities - Additional Information (Detail) - Marketable securities [Member]	6 Months Ended
	Jun. 30, 2014 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Gross unrealized losses on marketable securities	$ 3,000
Sales of available-for-sale securities	$ 0

Equity Incentive Plans - Schedule of Option Activity under Stock Plans and Related Information (Detail)	6 Months Ended
	Jun. 30, 2014 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
NUMBER OF SHARES, Beginning balance \| shares	2,236,997
NUMBER OF SHARES, Options granted \| shares	139,820
NUMBER OF SHARES, Options exercised \| shares	(159,161)
NUMBER OF SHARES, Options forfeited \| shares	(19,931)
NUMBER OF SHARES, Ending balance \| shares	2,197,725
NUMBER OF SHARES, Options exercisable \| shares	1,344,411
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options beginning balance \| $ / shares	$ 6.09
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options granted \| $ / shares	13.24
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options exercised \| $ / shares	5.23
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options forfeited \| $ / shares	7.19
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options ending balance \| $ / shares	6.60
WEIGHTED AVERAGE EXERCISE PRICE PER SHARE, Options exercisable \| $ / shares	$ 5.80

Equity Incentive Plans - Additional Information (Detail) - USD ($)	1 Months Ended	6 Months Ended
	Feb. 28, 2013	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options to purchase common stock outstanding, vested		2,174,205
Stock options to purchase common stock outstanding, weighted-average exercise price		$ 6.58
Options outstanding, weighted-average remaining contractual term		7 years 1 month 6 days
Weighted-average remaining contractual term options exercisable		6 years 2 months 12 days
Total unrecognized compensation expense related to nonvested employee and director stock options		$ 4,500,000
Unrecognized compensation expense expected to recognize, weighted-average period		2 years 3 months 18 days
Incremental stock-based compensation expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Incremental stock-based compensation expense	$ 157,000
Incremental stock-based compensation expenses [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option exercise period	18 months
Incremental stock-based compensation expenses [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option exercise period	20 months
2013 Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock available for future issuance		4,013,766

Equity Incentive Plans - Schedule of Stock-Based Compensation Expenses Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense recognized	$ 740	$ 411	$ 1,376	$ 1,035
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense recognized	369	188	729	400
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense recognized	$ 371	$ 223	$ 647	$ 635

Equity Incentive Plans - Schedule of Stock Option Assumptions (Detail) - Options [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)		6 years
Expected volatility	85.00%	85.00%	85.00%	85.00%
Risk-free interest rate		1.20%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	5 years 3 months 18 days		5 years 3 months 18 days	5 years
Risk-free interest rate	1.60%		1.60%	0.80%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (years)	6 years 1 month 6 days		6 years 1 month 6 days	6 years 1 month 6 days
Risk-free interest rate	1.90%		1.90%	1.20%

Collaborative Research and Development Agreements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended	6 Months Ended
	Mar. 14, 2014	Apr. 30, 2014	Feb. 28, 2014	Jun. 30, 2014	Jun. 30, 2014
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of common stock issued through public offering			3,450,000
Aggregate purchase price					$ 18,629
Bristol-Myers Squibb Company [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment		$ 20,000
Additional research funding received	$ 9,500
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, 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.
Deferred revenue recognition research period					5 years
Deferred revenue recognized under collaboration arrangement				$ 2,400	$ 2,400
Recognized revenue under collaboration arrangement				$ 1,900	2,100
Bristol-Myers Squibb Company [Member] \| Immuno-oncology collaboration [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional research funding received					$ 1,300
Number of common stock issued through public offering					994,352
Price per share				$ 21.16	$ 21.16
Aggregate purchase price					$ 21,000
Deferred revenue recognized under collaboration arrangement				$ 21,500	$ 21,500
GSK UK [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional research funding received		2,000
GSK UK [Member] \| 16 Equal quarterly payments [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Additional research funding received		$ 1,000