Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale, Amortized Cost	$ 991	$ 114,204
Accounts Receivable, Allowances	11,125	7,881
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	129,856	117,387
Intangible Assets, Accumulated Amortization	$ 8,519	$ 7,292
Treasury Stock at Cost, shares	31,393,480	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	71,977,753	68,698,675
Common Stock, Shares, Outstanding	40,584,273	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