Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Cash	$ 2,553		$ 2,553		$ 37
number of investments			1
Equity method investments	47,242		$ 47,242		40,171
Liabilities	0		0		0
Equity	74,871		74,871		20,230
Revenue	4,564	$ (1,843)	83,287	$ 24,083
Expenses	0	0	0	0
Net realized and unrealized gain (loss)	55	29	717	271
Net Income	4,619	(1,814)	84,004	24,354
Equity Method Investment, Summarized Financial Information, Performance And Management Fee Receivable	68,052		68,052		16,594
Equity Method Investment, Summarized Financial Information, Investments in Portfolio Funds	4,266		4,266		3,599
Other than Temporary Impairment Losses, Investments				7,100
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	9,946		9,946		7,589
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	22,627		22,627		12,699
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	3,827		3,827		11,000
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	2,474		2,474		1,167
Income (Loss) from Equity Method Investments	200		200
RCG Longview Debt Fund VI, LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	449		449		1,254
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	111		111		149
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	285		285		176
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	2,190		2,190		1,573
Triartisan ES Partners, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,026		1,026		1,500
Triartisan PFC Partners, LLC [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	999		999		0
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	65		65		131
RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	105		105		114
RCG LPP II GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	30		30		272
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,014		1,014		1,248
HCR Stafford Fund GP, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	682		682		0
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,081		1,081		968
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 2,700	$ (4,700)	$ 23,100	$ 7,200
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	1.00%		1.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	56.00%		56.00%
Clawback Obligation | RCG Longview Partners II, LLC
Schedule of Equity Method Investments [Line Items]
Contractual obligation	$ 6,500		$ 6,500		$ 6,500