Investments of Operating Entities and Consolidated Funds - Equity Method Investments - Quarterly (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Other investments	$ 153,334		$ 153,334		$ 167,464
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	(5,900)	$ 7,200	3,400	$ 17,500
Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Other Commitment	6,200		6,200		6,200
Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	42,574		42,574		61,443
Equity Method Investee, Exceeded Threshold for Income Test
Schedule of Equity Method Investments [Line Items]
Revenues	(3,216)	13,258	8,249	21,956
Expenses	(2,033)	(7,772)	(1,927)	(5,349)
Net realized and unrealized gains (losses)	(155)	(40)	51	165
Net Income	(5,404)	$ 5,446	6,373	$ 16,772
RCG Longview Debt Fund IV Management, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	464		464		676
RCG Longview Debt Fund V Partners, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	3,988		3,988		2,684
Healthcare Royalty GP, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	980		980		973
Healthcare Royalty GP II, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	1,260		1,260		1,125
Healthcare Royalty GP III, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	$ 71		$ 71		62
CBOE Stock Exchange, LLC
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.20%		7.20%
CBOE Stock Exchange, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	$ 568		$ 568		611
Starboard Value LP | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	30,079		30,079		48,772
RCG Longview Partners, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	0		0		237
RCG Longview Management, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	1,287		1,287		1,117
RCG Urban American, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	122		122		422
RCG Urban American Management, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	379		379		379
RCG Longview Equity Management, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	250		250		316
Urban American Real Estate Fund II, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	1,975		1,975		2,329
RCG Kennedy House, LLC | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	268		268		509
Equity Method Investee, Other | Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	883		883		1,231
RCG Longview Partners II, LLC | Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Other Commitment	$ 6,200		$ 6,200		$ 6,200
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%		20.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	55.00%		55.00%