Net Loss Per Share - Summary of Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share Basic And Diluted [Abstract]
Net loss	$ (37,850)	$ (63,228)	$ (41,059)	$ (39,764)	$ (33,340)	$ (27,139)	$ (142,137)	$ (100,243)
Accretion of redeemable convertible preferred stock				65,686				109,529
Net loss attributable to common stockholders	$ (37,850)			$ (105,450)			$ (142,137)	$ (209,772)
Weighted-average number of shares outstanding used to compute net loss per share attributable to common stockholders, basic and diluted	121,123,472			8,615,682			53,731,475	8,591,554
Net loss per share attributable to common stockholders, basic and diluted	$ (0.31)			$ (12.24)			$ (2.65)	$ (24.42)