Condensed Consolidated Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounts Receivable, Allowances	$ 2,492	$ 2,751
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	65,388	53,668
Intangible Assets, Accumulated Amortization	$ 32,115	$ 24,690
Treasury Stock at Cost, Shares	22,991,090	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	61,585,638	58,292,567
Common Stock, Shares Outstanding	38,594,548	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	83	24
Common Stock, Shares Outstanding	83	24