Loss Per Share - Schedule of Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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	$ (23,963)	$ (15,356)	$ (15,593)	$ (12,631)	$ (10,563)	$ (7,482)	$ (6,900)	$ (6,689)	$ (67,543)	$ (31,634)	$ (12,397)
Weighted average shares outstanding, basic and diluted	39,155	35,189	34,916	34,723	34,507	34,316	19,121	676	36,006	22,222	243
Net loss per share attributable to common stockholders, basic and diluted	$ (0.61)	$ (0.44)	$ (0.45)	$ (0.36)	$ (0.31)	$ (0.22)	$ (0.36)	$ (9.89)	$ (1.88)	$ (1.42)	$ (51.02)