Stockholders' Equity - Summary of Activity under 2008 Plan and 2018 Plan (Details) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares
Number of Options
Beginning balance (in shares) | shares	10,017,918
Options grants (in shares) | shares	2,762,183
Options exercised (in shares) | shares	(1,627,867)
Options cancelled (in shares) | shares	(491,824)
Ending balance (in shares) | shares	10,660,410	10,017,918
Vested and expected to vest (in shares) | shares	10,467,350
Exercisable (in shares) | shares	8,647,960
Weighted Average Exercise Price
Beginning balance (in dollars per share) | $ / shares	$ 10.52
Options granted (in dollars per share) | $ / shares	30.02
Options exercised (in dollars per share) | $ / shares	6.81
Options cancelled (in dollars per share) | $ / shares	21.55
Ending balance (in dollars per share) | $ / shares	15.63	$ 10.52
Vested and expected to vest, weighted average exercise price (in dollars per share) | $ / shares	15.46
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 12.31
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted Average Remaining Contractual Life (In Years)	6 years 9 months 29 days	6 years 5 months 12 days
Vested and expected to vest, weighted average remaining contractual life	6 years 9 months 14 days
Exercisable, weighted average remaining contractual life	6 years 2 months 23 days
Aggregate Intrinsic Value | $	$ 80,898	$ 198,097
Vested and expected to vest, aggregate intrinsic value | $	80,604
Exercisable, aggregate intrinsic value | $	$ 80,387