Consolidated Statements of Financial Condition (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts Receivable, Allowances	$ 6,037	$ 2,772
Furniture, Equipment and Leasehold Improvements, Accumulated Depreciation and Amortization	89,494	70,264
Intangible Assets, Accumulated Amortization	$ 41,217	$ 32,018
Treasury Stock at Cost, Shares	26,123,438	23,017,750
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	65,872,014	62,119,904
Common Stock, Shares Outstanding	39,748,576	39,102,154
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	86	82
Common Stock, Shares Outstanding	86	82