Fair Value Measurements for Operating Entities and Consolidated Funds - Unobservable Input Roll Forward (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2021		Jun. 30, 2020		Jun. 30, 2021		Jun. 30, 2020		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		$ 105,470		$ 51,480		$ 105,470		$ 51,480		$ 56,476	$ 59,967	$ 8,200	$ 7,835
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0		45,530	[1]	0		45,530	[1]
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		46,801		0		48,659		365
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0		2,100		4,651		2,100
Realized and unrealized gains (losses), asset		2,193		(150)		1,495		(150)
Change in Unrealized Gain (Loss), instruments still held, asset		2,193		2,411		(1,830)	[2]	2,411	[2]
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		41,600		14,816		41,600		14,816		22,706	23,786	17,510	17,466
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		3,409	[3]	0		3,409	[3]	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3		0		8	[4]	5,354	[5]	8	[4]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		6,246		0		9,472		1,798
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		5,086		3,163		6,473		3,755
Realized and unrealized gains (losses), asset		14,325		477		16,760		(685)
Change in Unrealized Gain (Loss), instruments still held, asset		14,820		(328)		16,498	[2]	(1,585)	[2]
Convertible Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		9,712		5,081		9,712		5,081		3,137	6,040	2,786	2,500
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		4,777		2,295		5,827		2,581
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		3,000		0		6,930		0
Realized and unrealized gains (losses), asset		4,798		0		4,775		0
Change in Unrealized Gain (Loss), instruments still held, asset		4,799		0		4,776	[2]	0	[2]
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		107		758		107		758		136	135	2,812	2,421
Balance Liability Value		0				0				415
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0		0		0		0
Liability, Transfers In		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, Liability, Transfers out of Level 3		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		12		98		82		520
Liability, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		55		1,938		103		1,962
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales		110
Realized and unrealized gains (losses), asset		14		(214)		(7)		(221)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(305)
Change in Unrealized Gain (Loss), instruments still held, asset		20		(214)		(29)	[2]	(202)	[2]
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)		0
Options (held long)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		244		337		244		337		241	251	332	336
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		0		0		0
Realized and unrealized gains (losses), asset		3		5		(7)		1
Change in Unrealized Gain (Loss), instruments still held, asset		3		5		(7)	[2]	1	[2]
Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		11,631		4,625		11,631		4,625		7,924	6,548	598	594
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		2,928	[3]	3,615	[1],[4]	2,928	[3]	3,615	[1],[4]
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		3,406		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		404		0		1,610		0
Realized and unrealized gains (losses), asset		1,183		412		359		416
Change in Unrealized Gain (Loss), instruments still held, asset		1,348		412		504	[2]	416	[2]
Trade Claims
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		5,719		7,155		5,719		7,155		5,905	8,713	9,183	7,083
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		0		0		0		1,044	[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		813		550		2,191		1,716
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		88		2,215		4,304		2,215
Realized and unrealized gains (losses), asset		(911)		(363)		(881)		(473)
Change in Unrealized Gain (Loss), instruments still held, asset		(2,923)		(383)		(1,902)	[2]	(493)	[2]
US Government Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value		0				0				522
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		291
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		231
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)		0
Term Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		12,563				12,563				12,424	12,623
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				322
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0				0
Realized and unrealized gains (losses), asset		139				(382)
Change in Unrealized Gain (Loss), instruments still held, asset		139				(382)	[2]
Private Equity Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		666		432		666		432		1,085	642	465	237
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		25		443		258
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		419		0		419		0
Realized and unrealized gains (losses), asset		0		(58)		0		(63)
Change in Unrealized Gain (Loss), instruments still held, asset		0		(58)		0		(63)
Consolidated Funds | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		0		0		0		0		0		4,393	4,393
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		0		4,000	[6]			4,000	[6]
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)				(393)
Change in Unrealized Gain (Loss), instruments still held, asset		0		0				0	[2]
Consolidated Funds | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		0		102,951		0		102,951		2,951	2,951	100,000	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		(4,000)	[3]	4,000	[6]	(4,000)	[3]	4,000	[6]
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		100,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0		0		0		0
Realized and unrealized gains (losses), asset		1,049		(1,049)		1,049		(1,049)
Change in Unrealized Gain (Loss), instruments still held, asset		0		(1,049)		0	[2]	(1,049)	[2]
Consolidated Funds | Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		0		5,858		0		5,858		5,329	5,806	5,611	5,567
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		3,777		0		4,447		0
Realized and unrealized gains (losses), asset		(1,552)		247		(1,359)		291
Change in Unrealized Gain (Loss), instruments still held, asset		0		247		0	[2]	291	[2]
Consolidated Funds | Term Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value				11,359				11,359				0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	[1]			11,149				11,149
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				210				210
Change in Unrealized Gain (Loss), instruments still held, asset				210				210
Options (held short)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value		3,619		2,177		3,619		2,177		3,297	3,915	1,802	2,920
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		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(322)		(375)		296		743
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)		322		375		(296)	[2]	(743)	[2]
Contingent liability payable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value		24,810		27,001		24,810		27,001		$ 19,579	37,952	28,658	30,896
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		4,253		11,312		5,653
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		(5,231)		(2,596)		1,830		(1,758)
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)		5,231		2,596		(1,830)	[2]	1,758	[2]
Corporate Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value				800				800				800
Balance Liability Value		0				0					704		1,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Liability, Transfers In						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				0
Liability, Purchases						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales						(399)		0
Realized and unrealized gains (losses), asset				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings						(305)		(200)
Change in Unrealized Gain (Loss), instruments still held, asset				0
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	[2]					0		(200)
US Government Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value				1,500				1,500				$ 1,500
Balance Liability Value		$ 0				0					$ 1,500		$ 1,950
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3				0
Liability, Transfers In						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers out of Level 3						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases				0
Liability, Purchases						0		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales						(1,569)		0
Realized and unrealized gains (losses), asset				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings						69		(450)
Change in Unrealized Gain (Loss), instruments still held, asset				$ 0
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	[2]					$ 0		$ (450)

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

[2] Unrealized gains/losses are reported in Investment income - Securities principal transactions, net in the accompanying condensed consolidated statements of operations

[3] Fair market value derived using models and private transaction levels.

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

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

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