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Significant Accounting Policies All Other - Quarterly (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Redeemable Noncontrolling Interest [Line Items]    
Total net assets of consolidated VIEs $ 3,900,000us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet $ 2,000,000us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssetsAndLiabilitiesNet
Deferred rent $ 12,343,000us-gaap_DeferredRentCredit $ 13,142,000us-gaap_DeferredRentCredit
Variable Interest Entity, Primary Beneficiary [Member]    
Redeemable Noncontrolling Interest [Line Items]    
Number of funds, Consolidated 2cown_NumberOfFunds
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember		  
Other investment companies    
Redeemable Noncontrolling Interest [Line Items]    
Number of funds, Consolidated 2cown_NumberOfFunds
/ us-gaap_InvestmentTypeAxis
= us-gaap_OtherInvestmentCompaniesMember		  
Investment Company    
Redeemable Noncontrolling Interest [Line Items]    
Number of funds, Consolidated 4cown_NumberOfFunds
/ us-gaap_InvestmentTypeAxis
= cown_InvestmentCompanyMember