Financial Information by Quarter (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Revenue from research contracts and other grants	$ 27	$ 1,059	$ 2,583	$ 6,088	$ 2,626	$ 4,168	$ 2,951	$ 4,474	$ 9,757	$ 14,219	$ 37,329
Operating expenses:
Research and development	30,832	21,852	20,641	20,906	25,076	21,087	12,984	13,762	94,231	72,909	52,402
General and administrative	13,917	12,882	12,213	10,303	10,399	8,014	7,054	6,127	49,315	31,594	14,630
Total operating expenses	44,749	34,734	32,854	31,209	35,475	29,101	20,038	19,889	143,546	104,503	67,032
Operating loss	(44,722)	(33,675)	(30,271)	(25,121)	(32,849)	(24,933)	(17,087)	(15,415)	(133,789)	(90,284)	(29,703)
Other income (loss):
Interest income (expense) and other, net	306	193	181	99	45	63	(19)	237	779	326	354
Gain (loss) on change in warrant valuation		4,256	(3,784)	(3,251)	23,984	(17,160)	(1,945)	(26,906)	(2,779)	(22,027)	(91,938)
Total other income (loss)	306	4,449	(3,603)	(3,152)	24,029	(17,097)	(1,964)	(26,669)	(2,000)	(21,701)	(91,584)
Net loss	$ (44,416)	$ (29,226)	$ (33,874)	$ (28,273)	$ (8,820)	$ (42,030)	$ (19,051)	$ (42,084)	$ (135,789)	$ (111,985)	$ (121,287)
Net loss per share-basic and diluted	$ (1.08)	$ (0.71)	$ (0.85)	$ (0.75)	$ (0.23)	$ (1.24)	$ (0.60)	$ (1.32)	$ (3.39)	$ (3.31)	$ (5.14)
Shares used in per share calculations-basic and diluted	41,304	41,066	39,862	37,821	37,596	33,943	31,984	31,813	40,026	33,850	23,602