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
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net loss	$ (17,820)	$ (14,762)
Net loss attributable to common stockholders, basic and diluted	$ (17,820)	$ (14,762)
Denominator:
Weighted average shares outstanding	35,889,835	31,428,564
Less: weighted average shares subject to repurchase	(62,600)	(339,254)
Weighted average shares used to compute basic and diluted net loss per share	35,827,235	31,089,310
Net loss per share attributable to common stockholders, basic and diluted	$ (0.50)	$ (0.47)