Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Payables and Accruals [Abstract]
Professional services and consulting and other vendor fees	$ 57,215	$ 38,796
Payroll and other employee related costs	28,834	39,820
Sales commissions	5,958	6,988
Finance lease liability	3,601	3,488
Restructuring	1,820	4,732	$ 314
Taxes other than income tax	1,057	2,954
Unrecognized tax benefits	2,040	2,039
Other	5,071	1,053
Total	$ 105,596	$ 99,870