Investments of Operating Entities and Consolidated Funds - Equity Method Investments (Details) (USD $)	6 Months Ended		3 Months Ended		6 Months Ended																																						3 Months Ended		6 Months Ended
	Jun. 30, 2014 cowenfund	Dec. 31, 2013	Jun. 30, 2014 Net Gains (Losses) on Securities, Derivatives and Other Investments	Jun. 30, 2013 Net Gains (Losses) on Securities, Derivatives and Other Investments	Jun. 30, 2014 Net Gains (Losses) on Securities, Derivatives and Other Investments	Jun. 30, 2013 Net Gains (Losses) on Securities, Derivatives and Other Investments	Jun. 30, 2014 Clawback Obligation	Dec. 31, 2013 Clawback Obligation	Jun. 30, 2014 CBOE Stock Exchange, LLC	Jun. 30, 2014 RCG Longview Partners II, LLC [Member] Clawback Obligation	Jun. 30, 2014 Equity Method Investments	Dec. 31, 2013 Equity Method Investments	Jun. 30, 2014 Equity Method Investments RCG Longview Debt Fund IV Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Debt Fund IV Management, LLC	Jun. 30, 2014 Equity Method Investments RCG Longview Debt Fund V Partners, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Debt Fund V Partners, LLC	Jun. 30, 2014 Equity Method Investments Healthcare Royalty GP, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP, LLC	Jun. 30, 2014 Equity Method Investments Healthcare Royalty GP II, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP II, LLC	Jun. 30, 2014 Equity Method Investments Healthcare Royalty GP III, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP III, LLC	Jun. 30, 2014 Equity Method Investments CBOE Stock Exchange, LLC	Dec. 31, 2013 Equity Method Investments CBOE Stock Exchange, LLC	Jun. 30, 2014 Equity Method Investments Starboard Value LP	Dec. 31, 2013 Equity Method Investments Starboard Value LP	Jun. 30, 2014 Equity Method Investments RCG Longview Partners, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Partners, LLC	Jun. 30, 2014 Equity Method Investments RCG Urban American, LLC	Dec. 31, 2013 Equity Method Investments RCG Urban American, LLC	Jun. 30, 2014 Equity Method Investments RCG Urban American Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Urban American Management, LLC	Jun. 30, 2014 Equity Method Investments RCG Longview Equity Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Equity Management, LLC	Jun. 30, 2014 Equity Method Investments Urban American Real Estate Fund II, LLC	Dec. 31, 2013 Equity Method Investments Urban American Real Estate Fund II, LLC	Jun. 30, 2014 Equity Method Investments RCG Kennedy House, LLC	Dec. 31, 2013 Equity Method Investments RCG Kennedy House, LLC	Jun. 30, 2014 Equity Method Investments Equity Method Investee, Other	Dec. 31, 2013 Equity Method Investments Equity Method Investee, Other	Jun. 30, 2014 Minimum	Jun. 30, 2014 Maximum	Jun. 30, 2014 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Jun. 30, 2013 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Jun. 30, 2014 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Jun. 30, 2013 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Summarized Financial Information, Revenue																																											$ 4,985,000	$ 2,948,000	$ 6,739,000	$ 9,522,000
Income (Loss) from Equity Method Investments			3,700,000	3,200,000	10,200,000	7,400,000
Equity Method Investment, Ownership Percentage									9.50%																																20.00%	55.00%
Equity Method Investment, Number of Entities that Parent Owns a Majority Interest	3
Other Commitment							6,200,000	6,200,000		6,200,000
Other investments	114,551,000	99,483,000									27,904,000	25,966,000	659,000	1,533,000	1,827,000	1,497,000	892,000	794,000	1,114,000	840,000	48,000	47,000	582,000	1,351,000	15,783,000	14,263,000	2,438,000	1,839,000	369,000	316,000	309,000	238,000	160,000	292,000	2,110,000	1,785,000	532,000	502,000	1,081,000	669,000
Equity Method Investment, Summarized Financial Information, Expenses																																											0	0	0	0
Equity Method Investment, Summarized Financial Information, Gain (Loss) on Investments																																											49,000	109,000	54,000	191,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)																																											5,034,000	3,057,000	6,793,000	9,713,000
Equity Method Investment, Other than Temporary Impairment	$ 800,000