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Investments of Operating Entities and Consolidated Funds -Real Estate (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Other investments $ 167,464us-gaap_OtherInvestments $ 99,483us-gaap_OtherInvestments
Real Estate Equity Investment    
Other investments 2,175us-gaap_OtherInvestments
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember		 2,088us-gaap_OtherInvestments
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember
Real Estate Equity Investment | RCG RE Manager, LLC    
Other investments $ 2,200us-gaap_OtherInvestments
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember
/ dei_LegalEntityAxis
= cown_RcgReManagerLlcMember		 $ 2,100us-gaap_OtherInvestments
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember
/ dei_LegalEntityAxis
= cown_RcgReManagerLlcMember