Net Loss per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net loss	$ (75,970)	$ (72,593)	$ (45,074)	$ (27,753)	$ (148,563)	$ (72,827)
Denominator:
Weighted-average common shares outstanding	71,094,199		50,153,160		69,792,730	48,053,186
Less: weighted-average common shares subject to vesting	(1,348,902)		(1,596,317)		(1,371,644)	(1,633,462)
Weighted-average common shares used to compute basic and diluted net loss per share	69,745,297		48,556,843		68,421,086	46,419,724
Net loss per share: basic and diluted	$ (1.09)		$ (0.93)		$ (2.17)	$ (1.57)