Condensed Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Revenue from research contracts and other grants	$ 2,583	$ 2,951	$ 8,671	$ 7,425
Operating expenses:
Research and development	20,641	12,984	41,547	26,746
General and administrative	12,213	7,054	22,516	13,181
Operating loss	(30,271)	(17,087)	(55,392)	(32,502)
Other income (loss):
Interest income (expense) and other, net	181	(19)	280	218
Loss on change in warrant valuation	(3,784)	(1,945)	(7,035)	(28,851)
Total other loss	(3,603)	(1,964)	(6,755)	(28,633)
Net loss	(33,874)	(19,051)	(62,147)	(61,135)
Other comprehensive income (loss):
Unrealized gain (loss) on available-for-sale securities	24	0	(35)	0
Total other comprehensive income (loss)	24	0	(35)	0
Comprehensive loss	$ (33,850)	$ (19,051)	$ (62,182)	$ (61,135)
Net loss per share - basic and diluted	$ (0.85)	$ (0.60)	$ (1.60)	$ (1.92)
Weighted average number of common stock outstanding for computing basic and diluted net loss per share	39,862	31,984	38,847	31,899