Net Loss per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (13,210)	$ (23,122)	$ (9,550)	$ (7,200)	$ (49,594)	$ (53,082)	$ (17,970)
Denominator:
Weighted-average common shares outstanding									36,357,336	3,885,508	3,374,609
Less: weighted-average common shares subject to repurchase									(1,739,099)	(2,057,246)	(2,510,626)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,163,412	42,838,098	42,533,641	9,488,352	2,208,065	1,949,258	1,693,150	1,452,215	34,618,237	1,828,262	863,983
Net loss per share, basic and diluted									$ (1.43)	$ (29.03)	$ (20.80)