Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (32,842)	$ (22,960)	$ (21,960)	$ (19,400)	$ (17,194)	$ (16,767)	$ (77,762)	$ (53,361)
Weighted-average common shares outstanding:
Basic and Diluted	44,517,134			38,034,216			41,314,044	34,696,835
Net loss per share, Basic and Diluted	$ (0.74)			$ (0.51)			$ (1.88)	$ (1.54)