Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 146	$ 1,597	$ 1,598	$ 1,597	$ 1,597	$ 146	$ 6,389	$ 6,861
Total operating expenses	12,762	12,570	12,378	16,342	15,033	14,676	14,611	15,562	54,052	59,882	172,168
Loss from operations	(12,762)	(12,570)	(12,378)	(16,196)	(13,436)	(13,078)	(13,014)	(13,965)	(53,906)	(53,493)	(165,307)
Preferred stock dividends	(342)	(6,074)	(5,462)	(5,120)	(4,999)	(4,903)	(4,865)	(4,171)	(16,998)	(18,938)	(7,123)
Settlement of a related party relationship	207,361	0	0	0
Net income (loss) applicable to common shareholders	$ 194,538	$ (18,659)	$ (17,493)	$ (21,540)	$ (18,272)	$ (17,604)	$ (17,727)	$ (19,658)	$ (53,117)	$ (54,323)	$ (165,297)
Net income (loss) per share, basic	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)					$ 0.96	$ (0.53)	$ (1.32)
Net income (loss) per share, diluted	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)					$ 0.96	$ (0.53)	$ (1.32)
Loss per share, basic and diluted					$ (0.12)	$ (0.13)	$ (0.13)	$ (0.15)