Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue from research contracts and other grants	$ 9,757us-gaap_ContractsRevenue	$ 14,219us-gaap_ContractsRevenue	$ 37,329us-gaap_ContractsRevenue
Operating expenses:
Research and development	94,231us-gaap_ResearchAndDevelopmentExpense	72,909us-gaap_ResearchAndDevelopmentExpense	52,402us-gaap_ResearchAndDevelopmentExpense
General and administrative	49,315us-gaap_GeneralAndAdministrativeExpense	31,594us-gaap_GeneralAndAdministrativeExpense	14,630us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	143,546us-gaap_OperatingExpenses	104,503us-gaap_OperatingExpenses	67,032us-gaap_OperatingExpenses
Operating loss	(133,789)us-gaap_OperatingIncomeLoss	(90,284)us-gaap_OperatingIncomeLoss	(29,703)us-gaap_OperatingIncomeLoss
Other income (loss):
Interest income and other, net	779us-gaap_OtherNonoperatingIncomeExpense	326us-gaap_OtherNonoperatingIncomeExpense	354us-gaap_OtherNonoperatingIncomeExpense
Loss on change in warrant valuation	(2,779)us-gaap_FairValueAdjustmentOfWarrants	(22,027)us-gaap_FairValueAdjustmentOfWarrants	(91,938)us-gaap_FairValueAdjustmentOfWarrants
Total other loss	(2,000)us-gaap_NonoperatingIncomeExpense	(21,701)us-gaap_NonoperatingIncomeExpense	(91,584)us-gaap_NonoperatingIncomeExpense
Net loss	(135,789)us-gaap_NetIncomeLoss	(111,985)us-gaap_NetIncomeLoss	(121,287)us-gaap_NetIncomeLoss
Other comprehensive loss:
Unrealized loss on short-term securities available-for-sale	(95)
Total other comprehensive loss	(95)
Comprehensive loss	$ (135,884)us-gaap_ComprehensiveIncomeNetOfTax	$ (111,985)us-gaap_ComprehensiveIncomeNetOfTax	$ (121,287)us-gaap_ComprehensiveIncomeNetOfTax
Net loss per share-basic and diluted	$ (3.39)us-gaap_EarningsPerShareBasicAndDiluted	$ (3.31)us-gaap_EarningsPerShareBasicAndDiluted	$ (5.14)us-gaap_EarningsPerShareBasicAndDiluted
Weighted average number of common stock outstanding for computing basic and diluted net loss per share	40,026	33,850	23,602

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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