Selected Quarterly Information (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
Quarterly Financial Information [Line Items]
Revenue	$ 340	$ 633	$ 200	$ 200	$ 200	$ 200	$ 200	$ 200	$ 1,373	$ 800	$ 800
Total operating expenses	10,936	12,575	11,377	9,984	6,919	9,315	18,496	23,783	44,872	58,513	102,969
Loss from operations	(10,596)	(11,942)	(11,177)	(9,784)	(6,719)	(9,115)	(18,296)	(23,583)	(43,499)	(57,713)	(102,169)
Change in fair value of warrants	194	5,847	5,600	82	(1,793)	(7,407)	(403)	10,788	11,723	1,185	6,050
Net (loss)	$ (10,401)	$ (6,093)	$ (5,576)	$ (9,711)	$ (8,903)	$ (16,713)	$ (18,692)	$ (12,799)	$ (31,781)	$ (57,107)	$ (96,132)
Loss per share, basic and diluted	$ (0.09)	$ (0.06)	$ (0.06)	$ (0.10)	$ (0.09)	$ (0.20)	$ (0.22)	$ (0.15)	$ (0.31)	$ (0.66)	$ (1.22)