Net Loss per Share Attributable to Common Stockholders - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net loss	$ (14,173)	$ (9,231)	$ (38,557)	$ (27,411)
Net loss attributable to common stockholders	$ (14,173)	$ (9,231)	$ (38,557)	$ (27,411)
Denominator
Weighted average shares outstanding	33,686,799	27,023,650	32,195,471	27,023,951
Less: weighted average shares subject to repurchase	(161,232)	(553,352)	(243,840)	(692,099)
Weighted average shares used to compute basic and diluted net loss per share	33,525,567	26,470,298	31,951,631	26,331,852
Net loss per share attributable to common stockholders, basic and diluted	$ (0.42)	$ (0.35)	$ (1.21)	$ (1.04)