Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information [Line Items]
Revenue	$ 1,597	$ 1,598	$ 1,697	$ 1,969	$ 1,919	$ 1,869	$ 272	$ 272	$ 6,861	$ 4,332	$ 1,373
Total operating expenses	12,708	12,512	132,939	14,009	11,401	20,035	14,497	78,499	172,168	124,432	44,872
Loss from operations	(11,111)	(10,914)	(131,242)	(12,040)	(9,482)	(18,166)	(14,225)	(78,227)	(165,307)	(120,100)	(43,499)
Change in derivative liabilities	(3,532)	(3,591)									11,723
Net (loss) applicable to common shareholders	$ (14,756)	$ (14,445)	$ (131,200)	$ (12,019)	$ (9,476)	$ (18,170)	$ (14,211)	$ (78,231)	$ (165,297)	$ (120,088)	$ (31,781)
Loss per share, basic and diluted	$ (0.11)	$ (0.11)	$ (1.01)	$ (0.09)	$ (0.07)	$ (0.14)	$ (0.11)	$ (0.69)	$ (1.32)	$ (0.96)	$ (0.31)