Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost	$ 0	$ 114,204
Accounts Receivable, Allowances	10,833	7,881
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	136,815	117,387
Intangible Assets, Accumulated Amortization	$ 8,793	$ 7,292
Treasury Stock at Cost, shares	31,409,356	29,522,665
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	72,014,728	68,698,675
Common Stock, Shares, Outstanding	40,605,372	39,176,010
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	53	84
Common Stock, Shares, Outstanding	53	84