Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale, Amortized Cost	$ 147,995	$ 402,824
Accounts Receivable, Allowances	2,017	5,372
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	146,412	139,572
Intangible Assets, Accumulated Amortization	$ 3,023	$ 2,932
Treasury Stock at Cost, shares	33,385,488	31,445,058
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	74,521,960	72,195,283
Common Stock, Shares, Outstanding	41,136,472	40,750,225
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	47	48
Common Stock, Shares, Outstanding	47	48