Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowances	$ 7,881	$ 6,037
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	117,387	89,494
Intangible Assets, Accumulated Amortization	$ 7,292	$ 41,217
Treasury Stock at Cost, shares	29,522,665	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	68,698,675	65,872,014
Common Stock, Shares Outstanding	39,176,010	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	84	86
Common Stock, Shares Outstanding	84	86