Net Income (Loss) per Share - Computation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss	$ (77,975)	$ (75,970)	$ (72,593)	$ 1,822	$ (45,074)	$ (27,753)	$ (226,538)	$ (71,005)
Denominator:
Weighted-average common shares outstanding	71,286,685			64,132,283			70,296,187	53,452,007
Less: weighted-average common shares subject to vesting	(1,176,547)			(1,533,090)			(1,305,897)	(1,599,760)
Weighted-average common shares used to compute basic net income (loss) per share	70,110,138			62,599,193			68,990,290	51,852,247
Dilutive effect of stock options				2,538,926
Dilutive effect of ESPP				7,588
Weighted-average common shares used to compute diluted net income (loss) per share	70,110,138			65,145,707			68,990,290	51,852,247
Net income (loss) per share: basic	$ (1.11)			$ 0.03			$ (3.28)	$ (1.37)
Net income (loss) per share: diluted	$ (1.11)			$ 0.03			$ (3.28)	$ (1.37)