Stock-Based Compensation - Stock Option Activity (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, number of options (in shares) | shares	3,655,945
Granted (in shares) | shares	2,490,171
Exercised (in shares) | shares	(203,893)
Forfeited (in shares) | shares	(199,416)
Expired (in shares) | shares	(27,660)
Ending balance, number of options (in shares) | shares	5,715,147	3,655,945
Weighted- Average Exercise Price
Beginning balance, weighted-average exercise price (in USD per share) | $ / shares	$ 12.09
Granted (in USD per share) | $ / shares	28.04
Exercised (in USD per share) | $ / shares	5.33
Forfeited (in USD per share) | $ / shares	22.32
Expired (in USD per share) | $ / shares	28.42
Ending balance, weighted-average exercise price (in USD per share) | $ / shares	$ 18.84	$ 12.09
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Exercisable (in shares) | shares	2,363,828
Exercisable, Weighted-Average Exercise Price (in USD per share) | $ / shares	$ 10.28
Remaining Contractual Term (Years)	8 years 6 months 25 days	8 years 9 months 10 days
Remaining contractual term, exercisable (years)	7 years 7 months 28 days
Aggregate Intrinsic Value | $	$ 44,093	$ 59,274
Intrinsic value, exercisable | $	$ 34,790