Commitments and Contingencies - Narrative (Details) ft² in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)		Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) ft²	Sep. 30, 2009 ft²
Operating Leased Assets [Line Items]
Area of leased property | ft²								122	50
Tenant improvement allowances							$ 15,200
Total leasehold improvements	$ 11,452		$ 11,452		$ 14,526	[1]	$ 22,300
Operating lease term								7 years
Reduction in letter of credit					$ 400
Rent expense	500	$ 500	1,600	$ 1,600
Letter of Credit
Operating Leased Assets [Line Items]
Letter of credit as a security deposit								$ 2,300
Reduction in letter of credit	$ 400		$ 400

[1] The condensed consolidated balance sheet as of December 31, 2020 has been derived from the audited financial statements as of that date.