Fair Value Measurements for Operating Entities and Consolidated Funds - Unobservable Input Roll Forward (Details) - USD ($)	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019		Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	$ 53,418,000		$ 7,835,000		$ 53,418,000		$ 7,835,000		$ 51,480,000	$ 7,835,000	$ 7,401,000	$ 5,168,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		45,530,000	[1]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	7,744,000	[2]	1,000,000	[3]	7,744,000	[2]	1,000,000	[3]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	5,282,000		0		5,647,000		3,243,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	3,770,000		0		5,870,000		1,000,000
Realized and unrealized gains (losses), asset	8,170,000		1,434,000		8,020,000		1,424,000
Change in Unrealized Gain (Loss), instruments still held, asset	2,926,000		2,295,000		5,337,000		2,285,000
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	19,456,000		13,313,000		19,456,000		13,313,000		14,816,000	17,466,000	11,006,000	9,850,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	3,000	[4]	0		29,000	[4],[5]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	4,000,000		2,579,000		5,798,000		16,737,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		480,000		3,712,000		12,952,000
Realized and unrealized gains (losses), asset	643,000		208,000		(67,000)		(322,000)
Change in Unrealized Gain (Loss), instruments still held, asset	562,000		210,000		(1,023,000)		(122,000)
Convertible Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	5,081,000		2,541,000		5,081,000		2,541,000		5,081,000	2,500,000	6,318,000	3,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		2,801,000	[3]	0		4,826,000	[2],[3]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0		1,049,000		2,581,000		16,021,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		2,072,000		0		11,764,000
Realized and unrealized gains (losses), asset	0		47,000		0		110,000
Change in Unrealized Gain (Loss), instruments still held, asset	0		4,000		0		41,000
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	520,000		2,073,000		520,000		2,073,000		758,000	2,421,000	775,000	0
Balance Liability Value	800,000				800,000				800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		20,000	[4]	0		20,000	[4]
Liability, Transfers In	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	312,000	[2]	0		312,000	[2]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	72,000		1,117,000		592,000		1,892,000
Liability, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0		1,938,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0
Realized and unrealized gains (losses), asset	2,000		161,000		(243,000)		161,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	0
Change in Unrealized Gain (Loss), instruments still held, asset	22,000		161,000		(180,000)		161,000
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	0
Options
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	351,000				351,000				337,000	336,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0				0
Realized and unrealized gains (losses), asset	14,000				15,000
Change in Unrealized Gain (Loss), instruments still held, asset	14,000				15,000
Warrants and Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	4,983,000		584,000		4,983,000		584,000		4,625,000	594,000	595,000	1,666,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	3,000	[4]	0		3,636,000	[1],[5]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		72,000		0		188,000
Realized and unrealized gains (losses), asset	355,000		61,000		753,000		(894,000)
Change in Unrealized Gain (Loss), instruments still held, asset	337,000		(12,000)		753,000		20,000
Trade Claims
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	9,390,000		7,062,000		9,390,000		7,062,000		7,587,000	7,320,000	10,488,000	5,543,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		1,044,000	[4],[5]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	2,690,000		1,871,000		4,663,000		6,965,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	648,000		5,354,000		2,863,000		5,506,000
Realized and unrealized gains (losses), asset	(239,000)		57,000		(774,000)		60,000
Change in Unrealized Gain (Loss), instruments still held, asset	(239,000)		57,000		(795,000)		57,000
US Government Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	1,500,000				1,500,000				1,500,000
Liability, Transfers In	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0
Liability, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	0
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	0
Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	12,111,000				12,111,000				11,359,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0				11,149,000	[1]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	245,000				245,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0				0
Realized and unrealized gains (losses), asset	507,000				717,000
Change in Unrealized Gain (Loss), instruments still held, asset	506,000				717,000
Consolidated Funds | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	0		4,393,000		0		4,393,000			4,393,000	24,322,000	24,314,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			19,929,000	[3]	4,000,000	[6]	19,929,000	[3]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0		0		0
Realized and unrealized gains (losses), asset			0		(393,000)		8,000
Change in Unrealized Gain (Loss), instruments still held, asset			0		0		8,000
Consolidated Funds | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	102,951,000		0		102,951,000		0		102,951,000	0	1,017,000	94,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		4,000,000	[6]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		94,000	[3]	0		94,000	[3]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0		0		100,000,000		407,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		957,000		0		958,000
Realized and unrealized gains (losses), asset	0		34,000		(1,049,000)		551,000
Change in Unrealized Gain (Loss), instruments still held, asset	0		0		(1,049,000)		0
Consolidated Funds | Convertible Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	75,538,000				75,538,000				0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	75,000,000				75,000,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0				0
Realized and unrealized gains (losses), asset	538,000				538,000
Change in Unrealized Gain (Loss), instruments still held, asset	538,000				538,000
Consolidated Funds | Warrants and Rights [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	5,904,000		5,465,000		5,904,000		5,465,000		5,858,000	5,567,000	6,274,000	5,279,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		670,000		0		1,758,000
Realized and unrealized gains (losses), asset	46,000		(139,000)		337,000		1,944,000
Change in Unrealized Gain (Loss), instruments still held, asset	46,000		(809,000)		337,000		190,000
Option
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	2,954,000		2,500,000		2,954,000		2,500,000		2,177,000	2,920,000	2,779,000	2,096,000
Liability, Transfers In	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0		0		0		0
Liability, Purchases	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0		0		0		4,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(777,000)		279,000		(34,000)		(408,000)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	777,000		(279,000)		35,000		408,000
Contingent liability payable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value	30,024,000		29,834,000		30,024,000		29,834,000		$ 27,001,000	30,896,000	29,536,000	3,070,000
Liability, Transfers In	0		0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3	0		0		0		0
Liability, Purchases	0		0		0		27,700,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales	0		0		5,653,000		1,234,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(3,023,000)		(298,000)		(4,781,000)		(298,000)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	3,023,000		298,000		4,781,000		298,000
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	800,000		1,200,000		800,000		1,200,000			1,000,000	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			2,525,000	[4]	0		2,525,000	[4]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0		0		0
Realized and unrealized gains (losses), asset			(1,325,000)		(200,000)		(1,325,000)
Change in Unrealized Gain (Loss), instruments still held, asset			(1,325,000)		(200,000)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)							(1,325,000)
US Government Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value	$ 1,500,000		2,250,000		1,500,000		2,250,000			$ 1,950,000	$ 0	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3			4,681,000	[4]	0		4,681,000	[4]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases			0		0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0		0		0
Realized and unrealized gains (losses), asset			(2,431,000)		(450,000)		(2,431,000)
Change in Unrealized Gain (Loss), instruments still held, asset			$ (2,431,000)		$ (450,000)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)							$ (2,431,000)

[1] The Company consolidated an operating entity which holds preferred stock, loans and warrants.

[2] The investments were converted to common stock.

[3] The entity in which the Company is invested completed an initial public offering

[4] The transfers between level 1 and level 3 are due to the change in the availability of observable inputs.

[5] The security stopped trading on an open market.

[6] The investment was involved in a reverse merger and preferred stock was converted to common shares.