Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenue								$ 146		$ 146	$ 6,389
Total operating expenses	$ 16,036	$ 13,448	$ 14,753	$ 13,621	$ 12,762	$ 12,570	$ 12,378	16,342
Loss from operations	(16,036)	(13,448)	(14,753)	(13,621)	(12,762)	(12,570)	(12,378)	(16,196)	$ (57,858)	(53,906)	(53,493)
Non-cash inducement warrant expense		(60,751)							60,751
Preferred stock dividends					(342)	(6,074)	(5,462)	(5,120)		(16,998)	(18,938)
Settlement of a related party relationship					207,361
Net income (loss) applicable to common shareholders	$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	$ 194,538	$ (18,659)	$ (17,493)	$ (21,140)	$ (117,796)	$ (53,117)	$ (54,323)
Net income (loss) per share, basic	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)	$ (0.70)	$ 0.96	$ (0.53)
Net income (loss) per share, diluted	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)	$ (0.70)	$ 0.96	$ (0.53)