Fixed Assets, Net - Components of Fixed Assets, Net (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 456,414	$ 415,537
Less: accumulated depreciation and amortization	(280,677)	(249,670)
Fixed assets, net	175,737	165,867
Computer and Communications Equipment [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	144,204	130,538
Software, Including Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	143,078	130,397
Leasehold Improvements and Other Fixed Assets [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 169,132	$ 154,602