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Fixed Assets, Net - Components of Fixed Assets, Net (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]    
Fixed assets, gross $ 557,550 $ 494,688
Less: accumulated depreciation and amortization (341,419) (305,341)
Fixed assets, net 216,131 189,347
Computer and Communications Equipment [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross 155,946 151,499
Software, Including Software Development Costs [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross 201,119 164,011
Leasehold Improvements and Other Fixed Assets [Member]    
Property, Plant and Equipment [Line Items]    
Fixed assets, gross $ 200,485 $ 179,178