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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net loss	$ (19,400)	$ (12,444)	$ (53,361)	$ (43,948)
Weighted-average common shares outstanding:
Basic and Diluted	38,034,216	32,591,550	34,696,835	30,729,752
Net loss per share, Basic and Diluted	$ (0.51)	$ (0.38)	$ (1.54)	$ (1.43)