Stock-Based Compensation - Summary of Stock Option Activity (Details) - Stock Options $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Options Outstanding
Outstanding, beginning of period (in shares) | shares	13,295,497
Granted (in shares) | shares	3,717,968
Exercised (in shares) | shares	(244,264)
Cancelled/Forfeited (in shares) | shares	(749,962)
Outstanding, end of period (in shares) | shares	16,019,239	13,295,497
Vested and exercisable (in shares) | shares	9,514,873
Weighted-Average Exercise Price
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 1.29
Granted (in dollars per share) | $ / shares	22.22
Exercised (in shares) | $ / shares	3.20
Cancelled/Forfeited (in dollars per share) | $ / shares	1.11
Outstanding, end of period (in dollars per share) | $ / shares	6.13	$ 1.29
Vested and exercisable (in dollars per share) | $ / shares	$ 0.55
Weighted-Average Remaining Life (Years)
Outstanding	7 years 7 months 28 days	7 years 11 months 26 days
Vested and exercisable	6 years 6 months
Aggregate Intrinsic Value
Outstanding | $	$ 507,736	$ 97,986
Vested and exercisable | $	$ 348,041