Condensed Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accounts Receivable, Allowances	$ 1,943	$ 6,037
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	108,382	89,494
Intangible Assets, Accumulated Amortization	48,115	41,217
Commitments and Contingencies (Note 17)
Treasury Stock at Cost, Shares	29,137,838	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	68,573,861	65,872,014
Common Stock, Shares Outstanding	39,436,023	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