Nature of Business - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 22, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Class Of Stock [Line Items]
Gross proceeds from common stock					$ 11,896
Net Loss		$ 26,027	$ 19,486	$ 62,745	$ 53,908	$ 84,000	$ 75,400	$ 62,300
Accumulated deficit		(439,766)		(439,766)		$ (377,021)
Cash, cash equivalents and marketable securities		$ 166,706		$ 166,706
Underwritten Public Offering
Class Of Stock [Line Items]
Shares issued	5,400,000
Public offering price per share	$ 14.00
Gross proceeds from common stock	$ 75,600
Underwriting discounts and commissions and other transaction expenses	$ 5,100