Stock Based Compensation - Stock Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Number of Shares Available for Issuance Under the Plan
Beginning balance (in shares)	35,500,603
Options and restricted stock units granted (in shares)	(3,743,376)
Options canceled (in shares)	4,039,561
Ending balance (in shares)	35,796,788	35,500,603
Number of Shares Outstanding Under the Plan
Beginning balance (in shares)	38,404,493
Options granted (in shares)	64,839
Options exercised (in shares)	(4,896,629)
Options canceled (in shares)	(3,390,960)
Ending balance (in shares)	30,181,743	38,404,493
Exercisable (in shares)	13,459,112	14,248,234
Unvested (in shares)	16,722,631	24,156,259
Weighted- Average Exercise Price
Beginning balance (in dollars per share)	$ 5.89
Options granted (in dollars per share)	12.58
Options exercised (in dollars per share)	0.65
Options canceled (in dollars per share)	1.22
Ending balance (in dollars per share)	7.24	$ 5.89
Exercisable, weighted-average exercise price (in dollars per share)	0.16	0.18
Unvested, weighted-average exercise price (in dollars per share)	$ 12.93	$ 9.25
Weighted-Average Remaining Contractual Term and Aggregate Intrinsic Value
Options outstanding, weighted-average remaining contractual term	7 years 3 months 21 days	8 years 3 months 7 days
Exercisable, weighted-average remaining contractual term	5 years 4 months 13 days	6 years 5 months 12 days
Unvested, weighted-average remaining contractual term	8 years 10 months 13 days	9 years 4 months 2 days
Options outstanding, aggregate intrinsic value	$ 172,203	$ 542,074
Exercisable, aggregate intrinsic value	129,935	282,364
Unvested, aggregate intrinsic value	$ 42,268	$ 259,710
Restricted stock units
Number of Plan shares outstanding
Beginning balance (in shares)	341,256
Restricted stock units granted (in shares)	3,678,537
Restricted stock units released (in shares)	(20,303)
Restricted stock units canceled (in shares)	(648,601)
Ending balance (in shares)	3,350,889	341,256
Weighted-Average Grant Date Fair Value per share
Beginning balance (in dollars per share)	$ 17.68
Granted (in dollars per share)	20.28
Vested (in dollars per share)	17.48
Forfeited (in dollars per share)	22.46
Ending balance (in dollars per share)	$ 19.61	$ 17.68