Investments of Operating Entities and Consolidated Funds - Equity Method Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Number of Entities that Parent Owns a Majority Interest	3
Other investments	$ 167,464,000	$ 99,483,000
Equity Method Investment, Other than Temporary Impairment	800,000
Net Gains (Losses) on Securities, Derivatives and Other Investments
Schedule of Equity Method Investments [Line Items]
Income (Loss) from Equity Method Investments	49,100,000	16,100,000	15,600,000
Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Other Commitment	6,200,000	6,200,000
Equity Method Investee, Exceeded Threshold for Income Test
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Assets	190,261,000	114,110,000
Equity Method Investment, Summarized Financial Information, Liabilities	21,037,000	25,309,000
Equity Method Investment, Summarized Financial Information, Equity Excluding Noncontrolling Interests	169,224,000	88,801,000
Equity Method Investment, Summarized Financial Information, Revenue	157,187,000	78,217,000	75,046,000
Equity Method Investment, Summarized Financial Information, Expenses	(46,563,000)	(49,334,000)	(42,663,000)
Equity Method Investment, Summarized Financial Information, Gain (Loss) on Investments	11,894,000	18,056,000	2,091,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	122,518,000	46,939,000	34,474,000
CBOE Stock Exchange, LLC
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	9.50%
RCG Longview Partners II, LLC | Clawback Obligation
Schedule of Equity Method Investments [Line Items]
Other Commitment	6,200,000
Equity Method Investments
Schedule of Equity Method Investments [Line Items]
Other investments	61,443,000	25,966,000
Equity Method Investments | RCG Longview Debt Fund IV Management, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	676,000	1,533,000
Equity Method Investments | RCG Longview Debt Fund V Partners, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	2,684,000	1,497,000
Equity Method Investments | Healthcare Royalty GP, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	973,000	794,000
Equity Method Investments | Healthcare Royalty GP II, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	1,125,000	840,000
Equity Method Investments | Healthcare Royalty GP III, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	62,000	47,000
Equity Method Investments | CBOE Stock Exchange, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	611,000	1,351,000
Equity Method Investments | Starboard Value LP
Schedule of Equity Method Investments [Line Items]
Other investments	48,772,000	14,263,000
Equity Method Investments | RCG Longview Partners, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	237,000	1,839,000
Equity Method Investments | RCG Longview Management, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	1,117,000	308,000
Equity Method Investments | RCG Urban American, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	422,000	316,000
Equity Method Investments | RCG Urban American Management, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	379,000	238,000
Equity Method Investments | RCG Longview Equity Management, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	316,000	292,000
Equity Method Investments | Urban American Real Estate Fund II, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	2,329,000	1,785,000
Equity Method Investments | RCG Kennedy House, LLC
Schedule of Equity Method Investments [Line Items]
Other investments	509,000	502,000
Equity Method Investments | Equity Method Investee, Other
Schedule of Equity Method Investments [Line Items]
Other investments	1,231,000	361,000
Minimum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	20.00%
Maximum
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	55.00%
Starboard Value A LP | Equity Method Investee, Exceeded Threshold for Income Test
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Cash	197,000	14,000
Equity Method Investment, Summarized Financial Information, Equity Excluding Noncontrolling Interests	97,345,000	27,429,000
Equity Method Investment, Summarized Financial Information, Performance Fee Receivable	93,610,000	25,214,000
Equity Method Investment, Summarized Financial Information, Investments in Portfolio Funds	3,538,000	2,201,000
Equity Method Investment, Summarized Financial Information, Total Liabilities	0	0
Starboard Value A LP | Operating Entities | Equity Method Investee, Exceeded Threshold for Income Test
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Performance Fee Revenues	90,905,000	23,098,000	20,448,000
Equity Method Investment, Summarized Financial Information, Gain (Loss) on Investments	734,000	283,000	254,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)	$ 91,639,000	$ 23,381,000	$ 20,702,000