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Investments of Operating Entities and Consolidated Funds - Derivatives (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mar. 10, 2014
Derivative [Line Items]        
Derivative Asset, Fair Value, Gross Asset   $ 10,075,000us-gaap_DerivativeFairValueOfDerivativeAsset    
Embedded cash conversion value 36,800,000us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability     35,700,000us-gaap_EmbeddedDerivativeFairValueOfEmbeddedDerivativeLiability
Call Option, Fair Value 36,800,000us-gaap_OptionIndexedToIssuersEquitySettlementAlternativesCashAtFairValue     35,700,000us-gaap_OptionIndexedToIssuersEquitySettlementAlternativesCashAtFairValue
Warrants issued 15,218,000us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued      
Cost of hedge transaction and warrant, net       20,500,000cown_Costofhedgetransactionandwarrantnet
Receivable on derivative contracts, at fair value 49,877,000us-gaap_DerivativeAssets 10,075,000us-gaap_DerivativeAssets    
Derivative Asset, Fair Value, Amount Offset Against Collateral 0us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral [1] 0us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral [1]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 2,588,000us-gaap_DerivativeAssetFairValueOfCollateral [2] 92,000us-gaap_DerivativeAssetFairValueOfCollateral [2]    
Derivative asset, net of offset 47,289,000us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral 9,983,000us-gaap_DerivativeFairValueOfDerivativeAssetAmountNotOffsetAgainstCollateral    
Derivative Liability, Fair Value, Gross Liability   7,674,000us-gaap_DerivativeFairValueOfDerivativeLiability    
Derivative liabilities 41,330,000us-gaap_DerivativeLiabilities 7,674,000us-gaap_DerivativeLiabilities    
Financial Instruments, Owned and Pledged as Collateral, at Fair Value 0us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAtFairValue 0us-gaap_FinancialInstrumentsOwnedAndPledgedAsCollateralAtFairValue    
Derivative Liability, Fair Value, Amount Offset Against Collateral 0us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral [1] 0us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral [1]    
Derivative Liability, Fair Value, Amount Not Offset Against Collateral 1,603,000us-gaap_DerivativeLiabilityFairValueOfCollateral [2] 826,000us-gaap_DerivativeLiabilityFairValueOfCollateral [2]    
Derivative Liability, net of offset 39,727,000us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral 6,848,000us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral    
Trading days for expiration 80 days      
Futures        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 75,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		 285,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		    
Derivative liabilities 33,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		 275,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		    
Derivative Asset, Notional Amount 3,041,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		 11,762,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		    
Derivative Liability, Notional Amount 2,213,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		 13,856,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember		    
Currency Forwards        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 310,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		 22,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		    
Derivative liabilities 0us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		 301,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		    
Derivative Asset, Notional Amount 23,961,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		 3,270,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		    
Derivative Liability, Notional Amount 0us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		 21,867,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeForwardMember		    
Equity Swaps        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 251,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 70,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		    
Derivative liabilities 1,603,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 525,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		    
Derivative Asset, Notional Amount 12,904,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 10,996,000us-gaap_DerivativeAssetNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		    
Derivative Liability, Notional Amount 18,352,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		 17,271,000us-gaap_DerivativeLiabilityNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SwapMember		    
Options        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 48,201,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_CallOptionMember		 [3] 9,698,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_CallOptionMember		 [3]    
Derivative Asset, Number of Instruments Held 367,441,000us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_CallOptionMember		 [3] 71,000us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_CallOptionMember		 [3]    
Put Option        
Derivative [Line Items]        
Derivative liabilities 39,694,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_PutOptionMember		 [4] 6,573,000us-gaap_DerivativeLiabilities
/ us-gaap_DerivativeByNatureAxis
= us-gaap_PutOptionMember		 [4]    
Derivative Liability, Number of Instruments Held 22,000us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_PutOptionMember		 [4] 38,000us-gaap_DerivativeLiabilityNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_PutOptionMember		 [4]    
Foreign Exchange Option [Member]        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 1,040,000us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeOptionMember		 0us-gaap_DerivativeAssets
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeOptionMember		    
Derivative Asset, Number of Instruments Held 32,200,000us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeOptionMember		 0us-gaap_DerivativeAssetNumberOfInstrumentsHeld
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForeignExchangeOptionMember		    
Receivables from Brokers-Dealers and Clearing Organizations        
Derivative [Line Items]        
Collateral posted 4,200,000us-gaap_CollateralAlreadyPostedAggregateFairValue
/ us-gaap_BalanceSheetLocationAxis
= cown_ReceivablesFromBrokersDealersAndClearingOrganizationsMember		 10,000,000us-gaap_CollateralAlreadyPostedAggregateFairValue
/ us-gaap_BalanceSheetLocationAxis
= cown_ReceivablesFromBrokersDealersAndClearingOrganizationsMember		    
Other Income        
Derivative [Line Items]        
Realized and unrealized gains/(losses) related to derivatives trading activities (500,000)us-gaap_GainLossOnDerivativeInstrumentsHeldForTradingPurposesNet
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherIncomeMember		 4,400,000us-gaap_GainLossOnDerivativeInstrumentsHeldForTradingPurposesNet
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherIncomeMember		 7,800,000us-gaap_GainLossOnDerivativeInstrumentsHeldForTradingPurposesNet
/ us-gaap_IncomeStatementLocationAxis
= us-gaap_OtherIncomeMember		  
Merger Master        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 78,000us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= cown_MergerMasterMember		 30,000us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= cown_MergerMasterMember		    
Derivative liabilities 58,000us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember		 136,000us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember		    
Common Stock Class A        
Derivative [Line Items]        
Class of Warrant or Right, Number of Securities Called by Warrants or Rights 28,048,786us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		      
Class of Warrant, Exercise Price of Warrants or Rights       $ 7.18us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember
Forward Contracts | Merger Master        
Derivative [Line Items]        
Derivative liabilities 0us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForwardContractsMember		 11,000us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForwardContractsMember		    
Equity Swaps | Merger Master        
Derivative [Line Items]        
Receivable on derivative contracts, at fair value 78,000us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SwapMember		 30,000us-gaap_DerivativeAssets
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SwapMember		    
Derivative liabilities $ 58,000us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SwapMember		 $ 125,000us-gaap_DerivativeLiabilities
/ dei_LegalEntityAxis
= cown_MergerMasterMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_SwapMember		    
[1] Includes financial instruments subject to enforceable master netting provisions that are permitted to be offset to the extent an event of default has occurred.
[2] Includes the amount of collateral held or posted.
[3] Options other include index, equity, commodity future and cash conversion options.
[4] Options include index, equity, commodity future and cash conversion options.