Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale, Amortized Cost	$ 402,824	$ 114,204
Accounts Receivable, Allowances	5,372	7,881	$ 6,037
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	139,572	117,387
Intangible Assets, Accumulated Amortization	$ 2,932	$ 7,292
Treasury Stock at Cost, shares	31,445,058	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	72,195,283	68,698,675
Common Stock, Shares, Outstanding	40,750,225	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	48	84
Common Stock, Shares, Outstanding	48	84