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	$ 113,398	$ 110,586
Accumulated depreciation and amortization	(48,833)	(45,764)
Fixed assets, net	64,565	64,822
Leasehold Improvements and Other Fixed Assets [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	79,121	77,313
Software, Including Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	17,827	17,395
Computer and Communications Equipment [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 16,450	$ 15,878