Consolidated Statements of Operations (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration and license revenue	$ 9,625us-gaap_LicenseAndServicesRevenue	$ 10,531us-gaap_LicenseAndServicesRevenue	$ 72,042us-gaap_LicenseAndServicesRevenue
Operating expenses:
Research and development	123,639us-gaap_ResearchAndDevelopmentExpense	79,286us-gaap_ResearchAndDevelopmentExpense	49,717us-gaap_ResearchAndDevelopmentExpense
General and administrative	23,552us-gaap_GeneralAndAdministrativeExpense	15,423us-gaap_GeneralAndAdministrativeExpense	11,469us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	147,191us-gaap_OperatingExpenses	94,709us-gaap_OperatingExpenses	61,186us-gaap_OperatingExpenses
(Loss) income from operations	(137,566)us-gaap_OperatingIncomeLoss	(84,178)us-gaap_OperatingIncomeLoss	10,856us-gaap_OperatingIncomeLoss
Interest and other income, net	441us-gaap_NonoperatingIncomeExpense	826us-gaap_NonoperatingIncomeExpense	510us-gaap_NonoperatingIncomeExpense
Net (loss) income	(137,125)us-gaap_NetIncomeLoss	(83,352)us-gaap_NetIncomeLoss	11,366us-gaap_NetIncomeLoss
Net (loss) income attributable to common stockholders:
Basic	(137,125)	(83,352)
Diluted	$ (137,125)	$ (83,352)
Net (loss) income per share attributable to common stockholders:
Basic	$ (3.19)us-gaap_EarningsPerShareBasic	$ (3.65)us-gaap_EarningsPerShareBasic
Diluted	$ (3.19)us-gaap_EarningsPerShareDiluted	$ (3.65)us-gaap_EarningsPerShareDiluted
Shares used to compute net (loss) income per share attributable to common stockholders:
Basic	42,977,463	22,842,443	1,350,939
Diluted	42,977,463	22,842,443	2,048,867

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