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Fixed Assets, Net - Components of Fixed Assets, Net (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]    
Fixed assets, gross $ 516,403 $ 494,688
Less: accumulated depreciation and amortization (319,497) (305,341)
Fixed assets, net 196,906 189,347
Computer and Communications Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross 154,171 151,499
Software, Including Software Development Costs [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross 175,280 164,011
Leasehold Improvements and Other Fixed Assets [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross $ 186,952 $ 179,178