Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information [Line Items]
Revenue	$ 1,919	$ 1,869	$ 272	$ 272	$ 340	$ 633	$ 200	$ 200	$ 4,332	$ 1,373	$ 800
Total operating expenses	11,401	20,035	14,497	78,499	10,936	12,575	11,377	9,984	124,432	44,872	58,513
Loss from operations	(9,482)	(18,166)	(14,225)	(78,227)	(10,596)	(11,942)	(11,177)	(9,784)	(120,100)	(43,499)	(57,713)
Change in fair value of warrants					194	5,847	5,600	82		11,723	1,185
Net (loss)	$ (9,476)	$ (18,170)	$ (14,211)	$ (78,231)	$ (10,401)	$ (6,093)	$ (5,576)	$ (9,711)	$ (120,088)	$ (31,781)	$ (57,107)
Loss per share, basic and diluted	$ (0.07)	$ (0.14)	$ (0.11)	$ (0.69)	$ (0.09)	$ (0.06)	$ (0.06)	$ (0.10)	$ (0.96)	$ (0.31)	$ (0.66)