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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (16,598)	$ (22,352)	$ (28,090)	$ (17,670)	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (84,710)	$ (49,594)
Denominator:
Weighted-average common shares outstanding									45,385,489	36,357,336
Less: weighted-average common shares subject to repurchase									(1,559,498)	(1,739,099)
Weighted-average number of shares used to compute basic and diluted net loss per share	44,056,407	43,939,281	43,797,718	43,508,592	43,163,412	42,838,098	42,533,641	9,488,352	43,825,991	34,618,237
Net loss per share, basic and diluted									$ (1.93)	$ (1.43)