Fixed Assets, Net - Components of Fixed Assets, Net (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 164,957	$ 137,229
Accumulated depreciation and amortization	(71,113)	(58,424)
Total	93,844	78,805
Leasehold Improvements and Other Fixed Assets [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	114,382	99,207
Software, Including Software Development Costs [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	29,598	21,417
Computer and Communications Equipment [Member]
Property, Plant and Equipment [Line Items]
Fixed assets, gross	$ 20,977	$ 16,605