Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts Receivable, Allowances	$ 3,426	$ 2,772
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	78,731	70,264
Accumulated Amortization	$ 36,616	$ 32,018
Treasury Stock at Cost, Shares	24,678,992	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	65,623,354	62,119,904
Common Stock, Shares Outstanding	40,944,362	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	86	82
Common Stock, Shares Outstanding	86	82