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	124 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, 2003	Dec. 31, 2013	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, 2013
Quarterly Financial Information [Line Items]
Revenue	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200
Total operating expenses	6,919	9,315	18,496	23,783	39,043	21,927	23,166	18,833		58,513	102,969	72,067									359,176
Loss from operations	(6,719)	(9,115)	(18,296)	(23,583)	(38,843)	(21,727)	(22,966)	(18,633)		(57,713)	(102,169)	(71,400)									(356,909)
Change in fair value of warrants	(1,793)	(7,407)	(403)	10,788	8,566	3,945	(650)	(5,811)		1,185	6,050	7,583									11,956
Net (loss)	$ (8,903)	$ (16,713)	$ (18,692)	$ (12,799)	$ (30,225)	$ (17,824)	$ (23,613)	$ (24,470)	$ (160)	$ (57,107)	$ (96,132)	$ (63,778)	$ (32,670)	$ (7,649)	$ (25,231)	$ (26,608)	$ (17,857)	$ (9,517)	$ (5,687)	$ (15,364)	$ (340,831)
Loss per share, basic and diluted	$ (0.09)	$ (0.20)	$ (0.22)	$ (0.15)	$ (0.37)	$ (0.23)	$ (0.30)	$ (0.32)		$ (0.66)	$ (1.22)	$ (0.97)								$ (3.75)