Investments of Operating Entities and Consolidated Funds - Equity Method Investments - (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Expense, equity method investments	$ 2,600	$ 900	$ 6,600	$ 3,500
Equity method investments	42,321		42,321		$ 41,099
Other than Temporary Impairment Losses, Investments			7,100
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	(4,700)	$ 4,100	7,200	$ 12,100
Surf House Ocean Views Holdings, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	7,478		7,478		14,179
Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	17,842		17,842		13,202
RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	10,273		10,273		9,284
RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,316		1,316		1,149
RCG Longview Debt Fund VI, LP
Schedule of Equity Method Investments [Line Items]
Equity method investments	971		971		0
Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	174		174		281
Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	197		197		148
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	1,562		1,562		764
RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	119		119		154
RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	114		114		114
RCG LPP II GP, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	272		272		487
RCG Park Liberty GP Member Manager, LLC
Schedule of Equity Method Investments [Line Items]
Equity method investments	1,018		1,018		428
Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 654		$ 654		578
Equity Method Investee, Exceeded Threshold for Income Test [Member]
Schedule of Equity Method Investments [Line Items]
Number of investments			1
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	13.00%		13.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	56.00%		56.00%
Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Contractual obligation					6,200
Clawback Obligation | RCG Longview Partners II, LLC
Schedule of Equity Method Investments [Line Items]
Contractual obligation	$ 6,200		$ 6,200		$ 6,200