Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Accounts Receivable, Allowances	$ 7,359	$ 6,037
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	101,600	89,494
Intangible Assets, Accumulated Amortization	45,816	41,217
Commitments and Contingencies (Note 17)
Treasury Stock at Cost, Shares	28,643,895	26,123,438
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	68,508,501	65,872,014
Common Stock, Shares Outstanding	39,864,606	39,748,576
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	86
Common Stock, Shares Outstanding	87	86