Stock-Based Compensation - Stock Options (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Employee and non-employee stock options
Stock Based Compensation
Shares available for grant, beginning balance	3,197,000
Additional shares authorized	3,446,000
Options granted (in shares)	(391,000)
Options forfeited (in shares)	35,000
RSUs granted (in shares)	(1,528,000)
RSUs forfeited/cancelled (in shares)	227,000
Shares available for grant, end balance	4,986,000	3,197,000
Number of shares
Outstanding, beginning balance (in shares)	6,707,000
Options granted (in shares)	391,000
Options exercised (in shares)	(1,051,000)
Options forfeited (in shares)	(35,000)
Outstanding, end balance (in shares)	6,012,000	6,707,000
Exercisable (in shares)	4,785,000
Vested and expected to vest (in shares)	5,936,000
Weighted-Average Exercise Price
Outstanding, beginning balance (in dollars per share)	$ 11.19
Granted (in dollars per share)	104.02
Exercised (in dollars per share)	10.26
Forfeited (in dollars per share)	13.78
Outstanding, end balance (in dollars per share)	17.37	$ 11.19
Exercisable (in dollars per share)	9.99
Vested and expected to vest (in dollars per share)	$ 17.00
Additional disclosures
Weighted average contractual term, options outstanding	5 years 7 months 17 days	6 years 14 days
Exercisable (in years)	4 years 11 months 23 days
Vested and expected to vest (in years)	5 years 7 months 6 days
Aggregate intrinsic value, options outstanding	$ 566,871	$ 592,468
Aggregate intrinsic value, options exercisable	485,498
Aggregate intrinsic value, vested and expected to vest	$ 561,788
Non-employee stock options
Stock Based Compensation
Options granted (in shares)	(32,053)
Number of shares
Options granted (in shares)	32,053
Additional disclosures
Options unvested (in shares)	1,459
Restricted stock units
Stock Based Compensation
RSUs granted (in shares)	(1,528,000)
RSUs forfeited/cancelled (in shares)	227,000