Financial Information by Quarter (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			365 Months Ended	401 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013
Income Statement [Abstract]
Revenues from license fees, grants and research contracts	$ 2,626	$ 4,168	$ 2,951	$ 4,474	$ 7,336	$ 7,574	$ 11,207	$ 11,212	$ 14,219	$ 37,329	$ 46,990		$ 187,767
Operating expenses:
Research and development	25,076	21,087	12,984	13,762	12,834	10,914	13,849	14,805	72,909	52,402	66,862		458,577
General and administrative	10,399	8,014	7,054	6,127	4,868	3,565	2,915	3,281	31,594	14,630	16,055		150,681
Operating loss	(32,849)	(24,933)	(17,087)	(15,415)	(10,366)	(6,905)	(5,557)	(6,874)	(90,284)	(29,703)	(35,927)		(450,952)
Other income (loss):
Interest income (expense) and other, net	45	63	(19)	237	83	67	107	96	326	354	587		9,849
Gain (loss) on change in warrant valuation	23,984	(17,160)	(1,945)	(26,906)	(51,784)	(42,716)	13,488	(10,926)	(22,027)	(91,938)	33,022		(88,995)
Net income (loss)	$ (8,820)	$ (42,030)	$ (19,051)	$ (42,084)	$ (62,067)	$ (49,554)	$ 8,038	$ (17,704)	$ (111,985)	$ (121,287)	$ (2,318)	$ (307,646)	$ (543,236)
Net income (loss) per share-basic					$ (2.36)	$ (2.17)	$ 0.36	$ (0.78)
Net income (loss) per share-basic and diluted	$ (0.23)	$ (1.24)	$ (0.60)	$ (1.32)					$ (3.31)	$ (5.14)	$ (0.11)
Net income (loss) per share-diluted					$ (2.36)	$ (2.17)	$ 0.35	$ (0.78)
Shares used in per share calculations-basic and diluted	37,596	33,943	31,984	31,813					33,850	23,602	21,599
Shares used in per share calculations-basic					26,313	22,824	22,624	22,624	33,850	23,602	21,599
Shares used in per share calculations-diluted					26,313	22,824	22,658	22,624