Furniture, Equipment and Leasehold Improvements - Composition of Furniture, Equipment and Leasehold Improvements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Furniture and Equipment	$ 31,715	$ 27,288
Leasehold Improvements	72,199	54,993
Computer and Technology-related Equipment	34,943	23,038
Total	138,857	105,319
Less: Accumulated Depreciation and Amortization	(70,264)	(53,668)
Furniture, Equipment and Leasehold Improvements, Net	$ 68,593	$ 51,651