Net Loss per Share - 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	$ (12,954)	$ (7,200)
Denominator:
Weighted-average common shares outstanding	11,598,421	3,549,791
Less: weighted-average common shares subject to repurchase	(2,110,069)	(2,097,576)
Weighted-average common shares used to compute basic and diluted net loss per share	9,488,352	1,452,215
Net loss per share: basic and diluted	$ (1.37)	$ (4.96)