Selected Quarterly Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								4 Months Ended	12 Months Ended									28 Months Ended	112 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, 2003	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Dec. 31, 2006	Dec. 31, 2005	Dec. 31, 2004	Dec. 31, 2005	Dec. 31, 2012
Quarterly Financial Information [Line Items]
Revenue	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200	$ 67		$ 800	$ 667									$ 1,467
Total operating expenses	39,043	21,927	23,166	18,833	16,617	14,409	13,048	27,993		102,969	72,067	24,546								300,663
Loss from operations	(38,843)	(21,727)	(22,966)	(18,633)	(16,417)	(14,209)	(12,848)	(27,926)		(102,169)	(71,400)	(24,546)								(299,196)
Change in fair value of warrants	8,566	3,945	(650)	(5,811)	3,160	13,388	2,115	(11,080)		6,050	7,583	(8,889)								10,771
Net loss	$ (30,225)	$ (17,824)	$ (23,613)	$ (24,470)	$ (13,244)	$ (802)	$ (10,724)	$ (39,008)	$ (160)	$ (96,132)	$ (63,778)	$ (32,670)	$ (7,649)	$ (25,231)	$ (26,608)	$ (17,857)	$ (9,517)	$ (5,687)	$ (15,364)	$ (283,724)
Loss per share, basic and diluted	$ (0.37)	$ (0.23)	$ (0.30)	$ (0.32)	$ (0.19)	$ (0.01)	$ (0.16)	$ (0.65)		$ (1.22)	$ (0.97)	$ (0.71)							$ (3.75)