Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (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
Earnings Per Share [Abstract]
Net loss	$ (28,611)	$ (32,842)	$ (22,960)	$ (21,960)	$ (20,652)	$ (19,400)	$ (17,194)	$ (16,767)	$ (106,373)	$ (74,013)	$ (58,125)
Weighted-average common shares outstanding:
Basic and Diluted									42,669,333	35,892,786	31,202,842
Net loss per share, Basic and Diluted	$ (0.61)	$ (0.74)	$ (0.58)	$ (0.56)	$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (2.49)	$ (2.06)	$ (1.86)