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Significant Accounting Policies - Revenue Recognition (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Revenue from External Customer [Line Items]      
Fees and Commissions $ 95.4us-gaap_FeesAndCommissions $ 79.7us-gaap_FeesAndCommissions $ 63.0us-gaap_FeesAndCommissions
Principal Transactions Revenue, Net 36.6us-gaap_PrincipalTransactionsRevenue 28.1us-gaap_PrincipalTransactionsRevenue 22.5us-gaap_PrincipalTransactionsRevenue
Revenue, Other Financial Services $ 8.1us-gaap_RevenueOtherFinancialServices $ 6.8us-gaap_RevenueOtherFinancialServices $ 5.7us-gaap_RevenueOtherFinancialServices
Hedge Funds      
Revenue from External Customer [Line Items]      
Incentive Fees, Percent Fee 20.00%cown_IncentiveFeesPercentFee
/ cown_IncentiveIncomeRevenueTypesAxis
= us-gaap_HedgeFundsMember
   
Alternative Solutions      
Revenue from External Customer [Line Items]      
Incentive Fees, Percent Fee 10.00%cown_IncentiveFeesPercentFee
/ cown_IncentiveIncomeRevenueTypesAxis
= cown_AlternativeSolutionsMember
   
Hedge Funds | Maximum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 2.00%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= us-gaap_HedgeFundsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Managed mutual funds | Maximum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 1.90%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= cown_ManagedmutualfundsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Managed mutual funds | Minimum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 1.35%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= cown_ManagedmutualfundsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Alternative Solutions | Maximum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 2.00%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= cown_AlternativeSolutionsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Real Estate Funds | Maximum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 1.50%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= us-gaap_RealEstateFundsMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
   
Real Estate Funds | Minimum      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 0.75%cown_AssetManagementFeesPercentFee
/ cown_ManagementFeeRevenueTypesAxis
= us-gaap_RealEstateFundsMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
   
Healthcare Royalty Partners | During investment period      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 2.00%cown_AssetManagementFeesPercentFee
/ cown_InvestmentPeriodAxis
= cown_DuringInvestmentPeriodMember
/ cown_ManagementFeeRevenueTypesAxis
= cown_HealthcareRoyaltyPartnersMember
   
Healthcare Royalty Partners | After investment period      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 2.00%cown_AssetManagementFeesPercentFee
/ cown_InvestmentPeriodAxis
= cown_AfterInvestmentPeriodMember
/ cown_ManagementFeeRevenueTypesAxis
= cown_HealthcareRoyaltyPartnersMember
   
Ramius Trading Strategies Managed Futures Fund | Maximum | Commodity trading advisory      
Revenue from External Customer [Line Items]      
Management Fees, Percent Fee 0.50%cown_AssetManagementFeesPercentFee
/ cown_BusinessTypeAxis
= cown_CommodityTradingAdvisoryMember
/ cown_ManagementFeeRevenueTypesAxis
= cown_RamiusTradingStrategiesManagedFuturesFundMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember