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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (51,853)	$ 1,822	$ (45,074)	$ (27,753)	$ (16,598)	$ (22,352)	$ (28,090)	$ (17,670)	$ (122,858)	$ (84,710)	$ (49,594)
Denominator:
Weighted-average common shares outstanding									56,354,059	45,385,489	36,357,336
Less: weighted-average common shares subject to repurchase									(1,566,941)	(1,559,498)	(1,739,099)
Weighted-average common shares used to compute basic and diluted net loss per share					44,056,407	43,939,281	43,797,718	43,508,592	54,787,118	43,825,991	34,618,237
Net loss per share, basic and diluted									$ (2.24)	$ (1.93)	$ (1.43)