Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowances	$ 4,273	$ 2,772
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	75,250	70,264
Accumulated Amortization	$ 34,317	$ 32,018
Treasury Stock at Cost, Shares	24,439,399	23,017,750
Class A [Member]
Common Stock, Par Value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, Shares Authorized	1,000,000,000	1,000,000,000
Common Stock, Shares Issued	65,391,119	62,119,904
Common Stock, Shares Outstanding	40,951,720	39,102,154
Class B [Member]
Common Stock, Par Value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, Shares Authorized	1,000,000	1,000,000
Common Stock, Shares Issued	87	82
Common Stock, Shares Outstanding	87	82