Financial Information by Quarter (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			353 Months Ended	389 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Revenues from license fees, grants and research contracts	$ 7,336	$ 7,574	$ 11,207	$ 11,212	$ 13,585	$ 7,524	$ 11,585	$ 14,296	$ 37,329	$ 46,990	$ 29,420		$ 173,548
Operating expenses:
Research and development	12,834	10,914	13,849	14,805	18,701	15,610	17,750	14,801	52,402	66,862	35,972		385,668
General and administrative	4,868	3,565	2,915	3,281	3,884	3,185	3,960	5,026	14,630	16,055	14,382		119,087
Operating loss	(10,366)	(6,905)	(5,557)	(6,874)	(9,000)	(11,271)	(10,125)	(5,531)	(29,703)	(35,927)	(20,934)		(360,668)
Other income (loss):
Interest income (expense) and other, net	83	67	107	96	147	199	151	90	354	587	259		9,523
Gain (loss) on change in warrant valuation	(51,784)	(42,716)	13,488	(10,926)	7,443	7,052	11,253	7,274	(91,938)	33,022	(11,502)		(66,968)
Net income (loss)	$ (62,067)	$ (49,554)	$ 8,038	$ (17,704)	$ (1,410)	$ (4,020)	$ 1,279	$ 1,833	$ (121,287)	$ (2,318)	$ (32,177)	$ (275,469)	$ (431,251)
Net income (loss) per share-basic	$ (2.36)	$ (2.17)	$ 0.36	$ (0.78)	$ (0.06)	$ (0.18)	$ 0.06	$ 0.10
Net income (loss) per share-diluted	$ (2.36)	$ (2.17)	$ 0.35	$ (0.78)	$ (0.06)	$ (0.18)	$ 0.06	$ 0.09
Shares used in per share calculations-basic	26,313	22,824	22,624	22,624	22,624	22,623	22,348	18,747	23,602	21,599	18,539
Shares used in per share calculations-diluted	26,313	22,824	22,658	22,624	22,624	22,623	23,153	20,214