Investments of Operating Entities and Consolidated Funds - Operating Entities - Equity Method Investment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 33,480		$ 33,480		$ 40,858
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	4,000	$ 2,700	10,500	$ 23,100
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	0		0		7,804
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	25,392		25,392		24,292
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,721		1,721		2,889
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	253		253		583
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	675		675		108
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	276		276		302
RCG Longview Debt Fund IV Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	331		331		331
Healthcare Royalty GP III, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	2,198		2,198		2,230
RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	105		105		105
HCR Stafford Fund GP, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	850		850		880
Liberty Harbor North
Schedule of Equity Method Investments [Line Items]
Equity method investments	292		292		292
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 1,387		$ 1,387		$ 1,042