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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (20,652)	$ (19,400)	$ (17,194)	$ (16,767)	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (74,013)	$ (58,125)	$ (57,280)
Weighted-average common shares outstanding:
Basic and Diluted									35,892,786	31,202,842	27,279,008
Net loss per share:
Basic and Diluted	$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (2.06)	$ (1.86)	$ (2.10)