Fixed Assets, Net - Components of Fixed Assets, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 479,281	$ 478,683
Less: accumulated depreciation and amortization	(279,932)	(262,659)
Fixed assets, net	199,349	216,024
Disposal Group, Held-for-sale, Not Discontinued Operations
Property, Plant and Equipment [Line Items]
Fixed assets, net	9,200
Computer and communications equipment
Property, Plant and Equipment [Line Items]
Fixed assets, gross	92,987	92,577
Software, including software development costs
Property, Plant and Equipment [Line Items]
Fixed assets, gross	284,287	265,082
Leasehold improvements and other fixed assets
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 102,007	$ 121,024