Selected Quarterly Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0							$ 146
Total operating expenses	22,774	20,321	18,606	18,660	$ 16,036	$ 13,448	$ 14,753	$ 13,621
Loss from operations	(22,774)	(20,321)	(18,606)	(18,660)	(16,036)	(13,448)	(14,753)	(13,621)	$ (80,361)	$ (57,858)	(53,906)
Non-cash inducement warrant expense						(60,751)				60,751
Net income (loss) applicable to common shareholders	$ (22,772)	$ (20,315)	$ (18,596)	$ (18,293)	$ (15,746)	$ (73,996)	$ (14,620)	$ (13,434)	$ (79,976)	$ (117,796)	$ (53,117)
Net income (loss) per share, basic	$ (0.11)	$ (0.10)	$ (0.09)	$ (0.09)	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ (0.38)	$ (0.70)	$ 0.96
Net income (loss) per share, diluted	$ (0.11)	$ (0.10)	$ (0.09)	$ (0.09)	$ (0.09)	$ (0.43)	$ (0.09)	$ (0.08)	$ (0.38)	$ (0.70)	$ 0.96