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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (21,971)	$ (11,542)	$ (13,075)	$ (10,692)	$ (58,125)	$ (57,280)	$ (24,690)
Weighted-average common shares outstanding:
Basic and Diluted									31,202,842	27,279,008	24,620,372
Net loss per share:
Basic and Diluted	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (0.81)	$ (0.42)	$ (0.48)	$ (0.39)	$ (1.86)	$ (2.10)	$ (1.00)