Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Statement [Abstract]
Revenue from research contracts and other grants		$ 2,583		$ 8,671
Operating expenses:
Research and development	$ 29,180	20,641	$ 68,345	41,547
General and administrative	12,927	12,213	35,624	22,516
Total operating expenses	42,107	32,854	103,969	64,063
Operating loss	(42,107)	(30,271)	(103,969)	(55,392)
Other income (loss):
Interest income and other, net	256	181	559	280
Loss on change in warrant valuation		(3,784)		(7,035)
Total other income (loss)	256	(3,603)	559	(6,755)
Net loss	(41,851)	(33,874)	(103,410)	(62,147)
Other comprehensive income (loss):
Unrealized (loss) gain on short-term securities - available-for-sale	(2)	24	76	(35)
Total other comprehensive (loss) income	(2)	24	76	(35)
Comprehensive loss	$ (41,853)	$ (33,850)	$ (103,334)	$ (62,182)
Loss per share - basic and diluted	$ (1.01)	$ (0.85)	$ (2.50)	$ (1.60)
Weighted average number of shares of common stock outstanding for computing basic and diluted net loss per share	41,357	39,862	41,341	38,847