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Related Party Transactions (Details) (USD $)
 6 Months Ended 12 Months Ended
Jun. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Apr. 30, 2011
Related Party Transaction [Line Items]          
Fees receivable, net of allowance   $ 46,498,000us-gaap_AccruedFeesAndOtherRevenueReceivable $ 45,067,000us-gaap_AccruedFeesAndOtherRevenueReceivable    
Real Estate Equity Investment          
Related Party Transaction [Line Items]          
Real estate loan participation agreements 50,000,000us-gaap_PaymentsForProceedsFromLoansAndLeases
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember		        
Interest Income, Related Party   500,000us-gaap_InterestIncomeRelatedParty
/ us-gaap_FairValueByAssetClassAxis
= cown_RealEstateEquityInvestmentMember		      
Employee Loans | Minimum          
Related Party Transaction [Line Items]          
Forgivable Loans, Vesting Period   1 year      
Employee Loans | Maximum          
Related Party Transaction [Line Items]          
Forgivable Loans, Vesting Period   3 years      
Affiliated Entity          
Related Party Transaction [Line Items]          
Fees receivable, net of allowance   5,100,000us-gaap_AccruedFeesAndOtherRevenueReceivable
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		 18,900,000us-gaap_AccruedFeesAndOtherRevenueReceivable
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		    
Affiliated Entity | Fees Payable          
Related Party Transaction [Line Items]          
Fees payable to related parties   100,000us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= cown_FeesPayableMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		 1,700,000us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent
/ us-gaap_BalanceSheetLocationAxis
= cown_FeesPayableMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		    
Affiliated Entity | Management Fee and Incentive Income          
Related Party Transaction [Line Items]          
Reimbursement to affiliated funds   700,000cown_RelatedPartyTransactionReimbursementsOfFeesRelatingToTransactionsWithRelatedParty
/ us-gaap_IncomeStatementLocationAxis
= cown_ManagementFeeandIncentiveIncomeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		 1,700,000cown_RelatedPartyTransactionReimbursementsOfFeesRelatingToTransactionsWithRelatedParty
/ us-gaap_IncomeStatementLocationAxis
= cown_ManagementFeeandIncentiveIncomeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		 1,500,000cown_RelatedPartyTransactionReimbursementsOfFeesRelatingToTransactionsWithRelatedParty
/ us-gaap_IncomeStatementLocationAxis
= cown_ManagementFeeandIncentiveIncomeMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_AffiliatedEntityMember		  
Employees          
Related Party Transaction [Line Items]          
Due from employees   6,100,000us-gaap_DueFromEmployees
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		 6,000,000us-gaap_DueFromEmployees
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		    
Forgivable Loan Balances   3,900,000cown_ForgivableLoanBalances
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		 3,800,000cown_ForgivableLoanBalances
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		    
Amortization on Forgivable Loans   4,400,000cown_AmortizationOnForgivableLoans
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		 3,200,000cown_AmortizationOnForgivableLoans
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		 1,900,000cown_AmortizationOnForgivableLoans
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= cown_EmployeesMember		  
Starboard Value LP          
Related Party Transaction [Line Items]          
Credit agreement with related party, maximum loan amount         3,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_EquityMethodInvesteeMember
Credit agreement with related party, loan receivable     1,500,000us-gaap_LoansAndLeasesReceivableRelatedParties
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_EquityMethodInvesteeMember		    
Starboard Value LP | LIBOR          
Related Party Transaction [Line Items]          
Credit agreement with related party, interest rate         3.75%us-gaap_LoansReceivableBasisSpreadOnVariableRate
/ cown_InterestRateBasisAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_EquityMethodInvesteeMember
Investor          
Related Party Transaction [Line Items]          
Due to Affiliate   $ 500,000us-gaap_DueToAffiliateCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_InvestorMember		 $ 400,000us-gaap_DueToAffiliateCurrentAndNoncurrent
/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis
= us-gaap_InvestorMember