Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Statement [Abstract]
Revenue from research contracts and other grants		$ 1,059		$ 9,730
Operating expenses:
Research and development	$ 36,673	21,852	$ 105,018	63,399
General and administrative	15,090	12,882	50,714	35,398
Total operating expenses	51,763	34,734	155,732	98,797
Operating loss	(51,763)	(33,675)	(155,732)	(89,067)
Other income (loss):
Interest (expense) income and other, net	(176)	193	383	473
Gain (loss) on change in warrant valuation		4,256		(2,779)
Total other (loss) income	(176)	4,449	383	(2,306)
Net loss	(51,939)	(29,226)	(155,349)	(91,373)
Other comprehensive income (loss):
Unrealized gain (loss) on short-term securities - available-for-sale	18	(21)	94	(55)
Total other comprehensive income (loss)	18	(21)	94	(55)
Comprehensive loss	$ (51,921)	$ (29,247)	$ (155,255)	$ (91,428)
Net loss per share — basic and diluted	$ (1.25)	$ (0.71)	$ (3.75)	$ (2.31)
Weighted average number of shares of common stock outstanding for computing basic and diluted net loss per share	41,565	41,066	41,416	39,595