Net Loss per Share - Antidilutive securities (Details) - shares	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Net Loss per Share
Antidilutive securities excluded from common stock	21,691,266	12,851,551
Redeemable convertible preferred stock
Net Loss per Share
Antidilutive securities excluded from common stock	19,185,183	11,788,435
Unvested restricted common stock
Net Loss per Share
Antidilutive securities excluded from common stock		23,710
Unvested shares from early exercises
Net Loss per Share
Antidilutive securities excluded from common stock	136,520	22,039
Stock options
Net Loss per Share
Antidilutive securities excluded from common stock	2,369,563	1,017,367