Investments of Operating Entities and Consolidated Funds - Equity Method Investments (Details) (USD $)	9 Months Ended		3 Months Ended		9 Months Ended																																								3 Months Ended		9 Months Ended
	Sep. 30, 2014 cowenfund	Dec. 31, 2013	Sep. 30, 2014 Net Gains (Losses) on Securities, Derivatives and Other Investments	Sep. 30, 2013 Net Gains (Losses) on Securities, Derivatives and Other Investments	Sep. 30, 2014 Net Gains (Losses) on Securities, Derivatives and Other Investments	Sep. 30, 2013 Net Gains (Losses) on Securities, Derivatives and Other Investments	Sep. 30, 2014 Clawback Obligation	Dec. 31, 2013 Clawback Obligation	Sep. 30, 2014 CBOE Stock Exchange, LLC	Sep. 30, 2014 RCG Longview Partners II, LLC [Member] Clawback Obligation	Sep. 30, 2014 Equity Method Investments	Dec. 31, 2013 Equity Method Investments	Sep. 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	Sep. 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	Sep. 30, 2014 Equity Method Investments Healthcare Royalty GP, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP, LLC	Sep. 30, 2014 Equity Method Investments Healthcare Royalty GP II, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP II, LLC	Sep. 30, 2014 Equity Method Investments Healthcare Royalty GP III, LLC	Dec. 31, 2013 Equity Method Investments Healthcare Royalty GP III, LLC	Sep. 30, 2014 Equity Method Investments CBOE Stock Exchange, LLC	Dec. 31, 2013 Equity Method Investments CBOE Stock Exchange, LLC	Sep. 30, 2014 Equity Method Investments Starboard Value LP	Dec. 31, 2013 Equity Method Investments Starboard Value LP	Sep. 30, 2014 Equity Method Investments RCG Longview Partners, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Partners, LLC	Sep. 30, 2014 Equity Method Investments RCG Longview Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Management, LLC	Sep. 30, 2014 Equity Method Investments RCG Urban American, LLC	Dec. 31, 2013 Equity Method Investments RCG Urban American, LLC	Sep. 30, 2014 Equity Method Investments RCG Urban American Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Urban American Management, LLC	Sep. 30, 2014 Equity Method Investments RCG Longview Equity Management, LLC	Dec. 31, 2013 Equity Method Investments RCG Longview Equity Management, LLC	Sep. 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	Sep. 30, 2014 Equity Method Investments RCG Kennedy House, LLC	Dec. 31, 2013 Equity Method Investments RCG Kennedy House, LLC	Sep. 30, 2014 Equity Method Investments Equity Method Investee, Other	Dec. 31, 2013 Equity Method Investments Equity Method Investee, Other	Sep. 30, 2014 Minimum	Sep. 30, 2014 Maximum	Sep. 30, 2014 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Sep. 30, 2013 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Sep. 30, 2014 Operating Companies [Member] Equity Method Investee, Exceeded Threshold for Income Test [Member]	Sep. 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																																													$ 3,432,000	$ 2,979,000	$ 10,171,000	$ 7,264,000
Income (Loss) from Equity Method Investments			7,200,000	5,200,000	17,500,000	12,600,000
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	130,584,000	99,483,000									30,935,000	25,966,000	728,000	1,533,000	2,049,000	1,497,000	917,000	794,000	1,297,000	840,000	52,000	47,000	435,000	1,351,000	19,766,000	14,263,000	237,000	1,839,000	1,034,000	308,000	396,000	316,000	344,000	238,000	250,000	292,000	2,208,000	1,785,000	533,000	502,000	689,000	361,000
Equity Method Investment, Summarized Financial Information, Expenses																																													0	0	0	0
Equity Method Investment, Summarized Financial Information, Gain (Loss) on Investments																																													34,000	43,000	89,000	95,000
Equity Method Investment, Summarized Financial Information, Net Income (Loss)																																													3,466,000	3,022,000	10,260,000	7,359,000
Equity Method Investment, Other than Temporary Impairment	$ 800,000