Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable, Allowances	$ 2,772	$ 2,751
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	70,264	53,668
Accumulated Amortization	$ 32,018	$ 24,690
Treasury Stock at Cost, Shares	23,017,750	19,101,711
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	62,119,904	58,292,567
Common Stock, Shares Outstanding	39,102,154	39,190,856
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	82	24
Common Stock, Shares Outstanding	82	24