Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Cash	$ 532		$ 532		$ 37
number of investments			1	0	0
Expense, equity method investments	2,000	$ 2,100	$ 4,700	$ 4,000
Equity method investments	54,164		54,164		$ 40,171
Liabilities	0		0		0
Equity	85,774		85,774		20,230
Revenue	10,144	21,356	78,723	25,926
Expenses	0	0	0	0
Net realized and unrealized gain (loss)	102	191	662	242
Net Income	10,246	21,547	79,385	26,168
Equity Method Investment, Summarized Financial Information, Performance And Management Fee Receivable	81,031		81,031		16,594
Equity Method Investment, Summarized Financial Information, Investments in Portfolio Funds	4,211		4,211		3,599
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	8,493		8,493		7,589
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	27,472		27,472		12,699
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	4,845		4,845		11,000
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	4,407		4,407		1,167
RCG Longview Debt Fund VI, LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,431		1,431		1,254
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	112		112		149
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	280		280		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	2,016		2,016		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	1,000		1,000		0
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	131		131		131
RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	107		107		114
RCG LPP II GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	94		94		272
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,248		1,248		1,248
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,171		1,171		968
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	$ 4,000	$ 8,000	$ 20,300	$ 11,900
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