Fixed Assets - Summary of Fixed Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	$ 81,577	$ 108,012	$ 103,493
Less: Accumulated depreciation and amortization	(70,194)	(90,823)	(82,831)
Fixed assets, net	11,383	17,189	20,662
Leasehold improvements
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	48,996	49,718	48,265
Furniture and fixtures
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	8,107	8,606	8,189
Equipment
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	15,897	35,293	33,501
Software
Property Plant And Equipment [Line Items]
Property, plant and equipment, gross	$ 8,577	$ 14,395	$ 13,538

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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