Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 1,597	$ 1,598	$ 1,597	$ 1,597	$ 1,597	$ 1,598	$ 1,697	$ 1,969	$ 6,389	$ 6,861	$ 4,332
Total operating expenses	15,033	14,676	14,611	15,562	12,708	12,512	132,939	14,009	59,882	172,168	124,432
Loss from operations	(13,436)	(13,078)	(13,014)	(13,965)	(11,111)	(10,914)	(131,242)	(12,040)	(53,493)	(165,307)	(120,100)
Preferred stock dividends	(4,999)	(4,903)	(4,865)	(4,171)	(3,532)	(3,591)			(18,938)	(7,123)
Net (loss) applicable to common shareholders	$ (18,272)	$ (17,604)	$ (17,727)	$ (19,658)	$ (14,756)	$ (14,445)	$ (131,200)	$ (12,019)	$ (54,323)	$ (165,297)	$ (120,088)
Loss per share, basic and diluted	$ (0.12)	$ (0.13)	$ (0.13)	$ (0.15)	$ (0.11)	$ (0.11)	$ (1.01)	$ (0.09)	$ (0.53)	$ (1.32)	$ (0.96)