Consolidated Statements of Financial Condition (Parenthetical) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Assets
Security Owned and Pledged as Collateral, Fair Value	$ 1,218,947,000	$ 941,595,000
Allowance for receivable from brokers	885,000	721,000
Allowance for receivable from customers	530,000	650,000
Allowance for fees receivable	3,348,000	2,620,000
Fixed assets, accumulated depreciation and amortization (in dollars)	40,670,000	32,846,000
Intangible assets, accumulated amortization (in dollars)	$ 37,884,000	$ 26,395,000
Stockholders' equity
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	120,750	120,750
Preferred Stock, Liquidation Preference, Value	$ 120,750,000	$ 120,750,000
Common stock, shares authorized	125,000,000
Portfolio Funds
Assets
Investment Owned, at Fair Value	$ 133,454,000	$ 114,504,000
Common Stock Class A
Stockholders' equity
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	62,500,000	62,500,000
Common stock, shares issued	49,465,491	47,215,938
Common stock, shares outstanding	26,845,628	28,610,357
Common stock, restricted shares	334,230	216,912
Treasury Stock, Shares	22,619,863	18,605,581
Common Stock Class B
Stockholders' equity
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	62,500,000	62,500,000
Common stock, shares issued	0	0
Common stock, shares outstanding	0	0