Five Prime Therapeutics (FPRX) 10-Q 14 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	7-May-15
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	FPRX
Entity Registrant Name	FIVE PRIME THERAPEUTICS INC
Entity Central Index Key	1175505
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,660,618

Condensed_Balance_Sheets

Condensed Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$14,482	$15,267
Marketable securities	202,659	133,787
Receivable from collaborative partners	133	410
Prepaid and other current assets	3,056	1,794
Total current assets	220,330	151,258
Property and equipment, net	3,830	3,794
Other long-term assets	409	579
Total assets	224,569	155,631
Current liabilities:
Accounts payable	904	1,096
Accrued personnel-related expenses	2,668	4,618
Other accrued liabilities	2,968	1,531
Deferred revenue, current portion	12,111	11,938
Deferred rent, current portion	655	632
Total current liabilities	19,306	19,815
Deferred revenue, long-term portion	48,537	48,628
Deferred rent, long-term portion	1,441	1,514
Other long-term liabilities	516	469
Commitments
Stockholders' equity:
Common stock	26	22
Preferred stock
Additional paid-in capital	354,746	274,180
Accumulated other comprehensive income	31	1
Accumulated deficit	-200,034	-188,998
Total stockholders' equity	154,769	85,205
Total liabilities and stockholders' equity	$224,569	$155,631

Condensed_Statements_of_Operat

Condensed Statements of Operations (USD $)	3 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$4,287	$3,546
Operating expenses:
Research and development	11,211	8,926
General and administrative	4,220	3,280
Total operating expenses	15,431	12,206
Loss from operations	-11,144	-8,660
Interest income	108	36
Other income (expense), net		-20
Net loss	($11,036)	($8,644)
Basic and diluted net loss per common share	($0.44)	($0.46)
Shares used to compute basic and diluted net loss per common share	25,072	18,841

Condensed_Statements_of_Compre

Condensed Statements of Comprehensive Loss (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net loss	($11,036)	($8,644)
Other comprehensive income:
Net unrealized gain on marketable securities	30	16
Comprehensive loss	($11,006)	($8,628)

Condensed_Statement_of_Cash_Fl

Condensed Statement of Cash Flows (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Operating activities
Net loss	($11,036)	($8,644)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	400	391
Stock-based compensation expense	991	636
Amortization of premium on marketable securities	534	211
Changes in operating assets and liabilities:
Receivable from collaborative partners	277	9
Prepaid, other current assets, and other long-term assets	-689	59
Accounts payable	-192	-1
Accrued personnel-related expenses	-1,950	-1,192
Deferred revenue	82	1,617
Deferred rent	-50	-138
Other accrued liabilities and other long-term liabilities	1,081	1,461
Net cash used in operating activities	-10,552	-5,591
Investing activities
Purchases of marketable securities	-93,876	-68,384
Maturities of marketable securities	24,500	18,500
Purchases of property and equipment	-436	-296
Net cash used in investing activities	-69,812	-50,180
Financing activities
Proceeds from public offering of common stock, net	78,693	40,104
Proceeds from the sale of common stock to collaborative partner		18,639
Proceeds from issuance of common stock under equity incentive plans	886	547
Net cash provided by financing activities	79,579	59,290
Net increase (decrease) in cash and cash equivalents	-785	3,519
Cash and cash equivalents at beginning of period	15,267	8,161
Cash and cash equivalents at end of period	$14,482	$11,680

Description_of_Business

Description of Business	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015 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, 2014 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, 2014 filed with the U.S. Securities and Exchange Commission.
	Follow-on Public Offering
	In January 2015, we closed an underwritten public offering of 3,829,994 shares of our common stock which included shares we issued pursuant to our underwriters’ exercise of their over-allotment option. We received net proceeds of $78.7 million, after underwriting discounts, structuring fees and offering expenses.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2	Summary of Significant Accounting Policies
	Use of Estimates
	The preparation of financial statements in conformity with 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.
	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 and 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. We do not have any Level 3 securities as of March 31, 2015.
	The following table summarizes, for assets recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		31-Mar-15
				BASIS OF FAIR VALUE
				MEASUREMENTS
		TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	10,271	$	10,271	$	—	$	—
	U.S. Treasury securities		202,659		202,659		—		—
	Total cash equivalents and marketable securities	$	212,930	$	212,930	$	—	$	—
		31-Dec-14
				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	$	—
	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 shares of common stock outstanding during the period.
	We excluded the following options and restricted stock awards, or RSAs, to purchase shares of common stock (in thousands) from the calculation of diluted net loss per share for all periods presented as the effect would have been antidilutive:
		Three Months Ended
		March 31,
		2015		2014
	Options and RSAs to purchase common stock		2,667		2,213
			2,667		2,213
	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	3 Months Ended
	Mar. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash Equivalents and Marketable Securities	3	Cash Equivalents and Marketable Securities
	The following is a summary of our cash equivalents and marketable securities (in thousands):
		31-Mar-15
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
	Money market funds	$	10,271		$	—	$	—		$	10,271
	U.S. Treasury securities		202,628			44		(13	)		202,659
			212,899			44		(13	)		212,930
	Less: cash equivalents		(10,271	)		—		—			(10,271	)
	Total marketable securities	$	202,628		$	44	$	(13	)	$	202,659
		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
	As of March 31, 2015, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):
		Amortized			Estimated
		Cost			Fair Value
	Debt securities maturing:
	In one year or less	$	115,824		$	115,831
	In one to two years		97,075			97,099
	Total marketable securities	$	212,899		$	212,930
	We determined that the gross unrealized losses of $13,000 on our marketable securities as of March 31, 2015 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 March 31, 2015. There were no sales of available-for-sale securities in any of the periods presented.

Equity_Incentive_Plans

Equity Incentive Plans	3 Months Ended
	Mar. 31, 2015
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
					WEIGHTED			WEIGHTED
					AVERAGE			AVERAGE
		NUMBER OF			EXERCISE PRICE			REMAINING
		SHARES			PER SHARE			CONTRACTUAL TERM
	Balance at December 31, 2014		2,684,812		$	7.85
	Options granted		23,088		$	24.31
	Options exercised		(143,426	)	$	6.23
	Options forfeited		(40,983	)	$	8.59
	Balance at March 31, 2015		2,523,491		$	8.08
	Options exercisable		1,388,801		$	6.32			5.93
	We have granted restricted stock awards, or RSAs to certain employees. 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. As of March 31, 2015, there were 24,000 shares of RSAs outstanding. We did not grant any RSAs during the first quarter of March 2015.
	As of March 31, 2015, there were 4,460,486 shares of common stock available for future issuance under our 2013 Omnibus Incentive Plan.
	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
		March 31,
		2015			2014
	Research and development	$	509		$	360
	General and administrative		482			276
	Total	$	991		$	636
	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
		March 31,
		2015			2014
	Expected term (years)	6.1				6.1
	Expected volatility		73	%		85	%
	Risk-free interest rate	1.5-1.7		%		1.8	%
	Expected dividend yield		0	%		0	%
	As of March 31, 2015, we had $6.7 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 2.9 years.

Employee_Benefit_Plans

Employee Benefit Plans	3 Months Ended
	Mar. 31, 2015
Postemployment Benefits [Abstract]
Employee Benefit Plans	5	Employee Benefit Plans
	We sponsor a 401(k) plan that stipulates that eligible employees can elect to contribute to the 401(k) plan, subject to certain limitations, up to the lesser of the statutory maximum or 100% of eligible compensation on a pre-tax basis. We pay the administrative costs for the plan.
	Effective January 1, 2015, we have elected to match employee contributions, or the Company Match, as permitted by the plan. We plan to make matching contributions on June 15 and December 15 each year in an amount equal to 50% of the amount contributed by the employee up to an annual maximum Company Match per employee equal to the lesser of (i) 4% of such employee’s compensation; or (ii) $6,000. We plan to deliver the Company Match through the issuance of shares of our common stock. We have accrued a 401(k) plan Company Match expense of $179,000 as of March 31, 2015.

Description_of_Business_Polici

Description of Business (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Unaudited Interim Financial Information	Unaudited Interim Financial Information
	The accompanying financial information as of March 31, 2015 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, 2014 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, 2014 filed with the U.S. Securities and Exchange Commission.
Use of Estimates	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.
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; 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.
	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. We do not have any Level 3 securities as of March 31, 2015.
	The following table summarizes, for assets recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		31-Mar-15
				BASIS OF FAIR VALUE
				MEASUREMENTS
		TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	10,271	$	10,271	$	—	$	—
	U.S. Treasury securities		202,659		202,659		—		—
	Total cash equivalents and marketable securities	$	212,930	$	212,930	$	—	$	—
		31-Dec-14
				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	$	—
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 shares of common stock outstanding during the period.
	We excluded the following options and restricted stock awards, or RSAs, to purchase shares of common stock (in thousands) from the calculation of diluted net loss per share for all periods presented as the effect would have been antidilutive:
		Three Months Ended
		March 31,
		2015		2014
	Options and RSAs to purchase common stock		2,667		2,213
			2,667		2,213
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_Account1

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Summary of Assets Recorded at Fair Value	The following table summarizes, for assets recorded at fair value, the respective fair values and the classifications by level of input within the fair value hierarchy defined above (in thousands):
		31-Mar-15
				BASIS OF FAIR VALUE
				MEASUREMENTS
		TOTAL		LEVEL 1		LEVEL 2		LEVEL 3
	Assets
	Money market funds	$	10,271	$	10,271	$	—	$	—
	U.S. Treasury securities		202,659		202,659		—		—
	Total cash equivalents and marketable securities	$	212,930	$	212,930	$	—	$	—
		31-Dec-14
				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	$	—
Options and Restricted Stock Awards Excluded from Calculation of Diluted Net Loss Per Share	We excluded the following options and restricted stock awards, or RSAs, to purchase shares of common stock (in thousands) from the calculation of diluted net loss per share for all periods presented as the effect would have been antidilutive:
		Three Months Ended
		March 31,
		2015		2014
	Options and RSAs to purchase common stock		2,667		2,213
			2,667		2,213

Cash_Equivalents_and_Marketabl1

Cash Equivalents and Marketable Securities (Tables)	3 Months Ended
	Mar. 31, 2015
Cash And Cash Equivalents [Abstract]
Summary of Cash Equivalents and Marketable Securities	The following is a summary of our cash equivalents and marketable securities (in thousands):
		31-Mar-15
		AMORTIZED			UNREALIZED		UNREALIZED			ESTIMATED
		COST BASIS			GAINS		LOSSES			FAIR VALUE
	Money market funds	$	10,271		$	—	$	—		$	10,271
	U.S. Treasury securities		202,628			44		(13	)		202,659
			212,899			44		(13	)		212,930
	Less: cash equivalents		(10,271	)		—		—			(10,271	)
	Total marketable securities	$	202,628		$	44	$	(13	)	$	202,659
		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
Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Securities by Contractual Maturity	As of March 31, 2015, the amortized cost and estimated fair value of our available-for-sale securities by contractual maturity are shown below (in thousands):
		Amortized			Estimated
		Cost			Fair Value
	Debt securities maturing:
	In one year or less	$	115,824		$	115,831
	In one to two years		97,075			97,099
	Total marketable securities	$	212,899		$	212,930

Equity_Incentive_Plans_Tables

Equity Incentive Plans (Tables)	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Option Activity under Stock Plans and Related Information	The following table summarizes option activity under our stock plans and related information:
		OPTIONS OUTSTANDING
					WEIGHTED			WEIGHTED
					AVERAGE			AVERAGE
		NUMBER OF			EXERCISE PRICE			REMAINING
		SHARES			PER SHARE			CONTRACTUAL TERM
	Balance at December 31, 2014		2,684,812		$	7.85
	Options granted		23,088		$	24.31
	Options exercised		(143,426	)	$	6.23
	Options forfeited		(40,983	)	$	8.59
	Balance at March 31, 2015		2,523,491		$	8.08
	Options exercisable		1,388,801		$	6.32			5.93
Schedule of Stock-Based Compensation Expenses Recognized	Total stock-based compensation expense recognized was as follows (in thousands)
		Three Months Ended
		March 31,
		2015			2014
	Research and development	$	509		$	360
	General and administrative		482			276
	Total	$	991		$	636
Schedule of Stock Option Weighted-Average 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
		March 31,
		2015			2014
	Expected term (years)	6.1				6.1
	Expected volatility		73	%		85	%
	Risk-free interest rate	1.5-1.7		%		1.8	%
	Expected dividend yield		0	%		0	%

Description_of_Business_Additi

Description of Business - Additional Information (Detail) (USD $)	3 Months Ended		1 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Jan. 31, 2015
	Segment
Description Of Business [Line Items]
Number of operating segment	1
Proceeds from the sale of common stock to collaborative partner		$18,639
Underwritten Public Offering [Member]
Description Of Business [Line Items]
Issuance of stock, Shares			3,829,994
Proceeds from the sale of common stock to collaborative partner			$78,700

Summary_of_Significant_Account2

Summary of Significant Accounting Policies - Summary of Assets Recorded at Fair Value (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Money market funds [Member]
Assets
Assets	$10,271	$9,996
U.S. Treasury securities [Member]
Assets
Assets	202,659	130,786
U.S. government agency securities [Member]
Assets
Assets		3,001
Total cash equivalents and marketable securities [Member]
Assets
Assets	212,930	143,783
Level 1 [Member] | Money market funds [Member]
Assets
Assets	10,271	9,996
Level 1 [Member] | U.S. Treasury securities [Member]
Assets
Assets	202,659	130,786
Level 1 [Member] | Total cash equivalents and marketable securities [Member]
Assets
Assets	212,930	140,782
Level 2 [Member] | U.S. government agency securities [Member]
Assets
Assets		3,001
Level 2 [Member] | Total cash equivalents and marketable securities [Member]
Assets
Assets		$3,001

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Options and Restricted Stock Awards Excluded from Calculation of Diluted Net Loss Per Share (Detail)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from calculation of diluted net (loss) income per share	2,667	2,213
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	2,667	2,213

Cash_Equivalents_and_Marketabl2

Cash Equivalents and Marketable Securities - Summary of Cash Equivalents and Marketable Securities (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	$202,628	$133,786
UNREALIZED GAINS	44	18
UNREALIZED LOSSES	-13	-17
Marketable securities	202,659	133,787
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	10,271	9,996
Marketable securities	10,271	9,996
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	202,628	130,785
UNREALIZED GAINS	44	18
UNREALIZED LOSSES	-13	-17
Marketable securities	202,659	130,786
Marketable securities including cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	212,899	143,782
UNREALIZED GAINS	44	18
UNREALIZED LOSSES	-13	-17
Marketable securities	212,930	143,783
U.S. government agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS		3,001
Marketable securities		3,001
Cash equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
AMORTIZED COST BASIS	10,271	9,996
Marketable securities	$10,271	$9,996

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 $)	Mar. 31, 2015
In Thousands, unless otherwise specified
Amortized Cost And Fair Value Debt Securities [Abstract]
Amortized Cost, In one year or less	$115,824
Amortized Cost, In one to two years	97,075
Total marketable securities, Amortized Cost	212,899
Estimated Fair Value, In one year or less	115,831
Estimated Fair Value, In one to two years	97,099
Total marketable securities, Estimated Fair Value	$212,930

Cash_Equivalents_and_Marketabl4

Cash Equivalents and Marketable Securities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Investments Debt And Equity Securities [Abstract]
Gross unrealized losses on marketable securities	$13,000
Sales of available-for-sale securities	$0

Equity_Incentive_Plans_Schedul

Equity Incentive Plans - Schedule of Option Activity under Stock Plans and Related Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Beginning balance	2,684,812
Number of Shares, Options granted	23,088
Number of Shares, Options exercised	-143,426
Number of Shares, Options forfeited	-40,983
Number of Shares, Ending balance	2,523,491
Number of Shares, Options exercisable	1,388,801
Weighted-Average Exercise Price Per Share, Options beginning balance	$7.85
Weighted-Average Exercise Price Per Share, Options granted	$24.31
Weighted-Average Exercise Price Per Share, Options exercised	$6.23
Weighted-Average Exercise Price Per Share, Options forfeited	$8.59
Weighted-Average Exercise Price Per Share, Options ending balance	$8.08
Weighted-Average Exercise Price Per Share, Options exercisable	$6.32
Weighted-Average Remaining Contractual Terms, Options exercisable	5 years 11 months 5 days

Equity_Incentive_Plans_Additio

Equity Incentive Plans - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, except Share data, unless otherwise specified	Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized compensation expense related to nonvested employee and director stock options	$6.70
Unrecognized compensation expense expected to recognize, weighted-average period	2 years 10 months 24 days
2013 Omnibus Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares of common stock reserved for future issuance	4,460,486
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding shares of RSA	24,000
Granted shares of RSA	0

Equity_Incentive_Plans_Schedul1

Equity Incentive Plans - Schedule of Stock-Based Compensation Expenses Recognized (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$991	$636
Research and development [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	509	360
General and administrative [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Stock-based compensation expense recognized	$482	$276

Equity_Incentive_Plans_Schedul2

Equity Incentive Plans - Schedule of Stock Option Assumptions (Detail) (Options [Member])	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
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
Expected volatility	73.00%	85.00%
Risk-free interest rate		1.80%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.50%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.70%

Employee_Benefit_Plans_Additio

Employee Benefit Plans - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended
	Jan. 02, 2015	Mar. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Eligible employees contribution percentage		100.00%
Defined contribution plan, employer matching contribution, percent of match	50.00%
Defined contribution plan, employer matching contribution, percent of employees' gross pay	4.00%
Defined contribution plan, employer matching contribution, amount	$6,000
Defined contribution plan, accrued company match expense		$179,000