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Income Taxes Deferred Tax Assets and Liabilities (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Deferred tax assets, net of valuation allowance      
Net operating loss $ 94,628,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards $ 130,562,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwards  
Deferred compensation 56,069,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits 22,538,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefits  
Goodwill 8,434,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets 9,937,000us-gaap_DeferredTaxAssetsGoodwillAndIntangibleAssets  
Foreign tax credits 2,668,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign 1,820,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsForeign  
Acquired lease liability 5,164,000cown_DeferredTaxAssetsAcquiredLeaseLiability 3,336,000cown_DeferredTaxAssetsAcquiredLeaseLiability  
Deferred Tax Assets, Other 2,394,000us-gaap_DeferredTaxAssetsOther 4,081,000us-gaap_DeferredTaxAssetsOther  
Total deferred tax assets 169,357,000us-gaap_DeferredTaxAssetsGross 172,274,000us-gaap_DeferredTaxAssetsGross  
Valuation allowance (2,263,000)us-gaap_DeferredTaxAssetsValuationAllowance (148,816,000)us-gaap_DeferredTaxAssetsValuationAllowance  
Deferred tax assets, net of valuation allowance 167,094,000us-gaap_DeferredTaxAssetsNet 23,458,000us-gaap_DeferredTaxAssetsNet  
Deferred tax liabilities      
Basis difference on investments (15,352,000)us-gaap_DeferredTaxLiabilitiesInvestments (15,352,000)us-gaap_DeferredTaxLiabilitiesInvestments  
Unrealized gains on investments (21,739,000)us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities (6,354,000)us-gaap_DeferredTaxLiabilitiesUnrealizedGainsOnTradingSecurities  
Intangible assets (537,000)us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets (1,177,000)us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets  
Deferred Tax Liabilities, Other (66,000)us-gaap_DeferredTaxLiabilitiesOther (602,000)us-gaap_DeferredTaxLiabilitiesOther  
Total deferred tax liabilities (37,694,000)us-gaap_DeferredIncomeTaxLiabilities (23,485,000)us-gaap_DeferredIncomeTaxLiabilities  
Deferred tax assets, net 129,400,000us-gaap_DeferredTaxAssetsLiabilitiesNet 0us-gaap_DeferredTaxAssetsLiabilitiesNet  
Valuation Allowance, Amount 2,263,000us-gaap_DeferredTaxAssetsValuationAllowance 148,816,000us-gaap_DeferredTaxAssetsValuationAllowance  
Deferred Income Tax Expense (Benefit) 128,105,000us-gaap_DeferredIncomeTaxExpenseBenefit (28,000)us-gaap_DeferredIncomeTaxExpenseBenefit (12,000)us-gaap_DeferredIncomeTaxExpenseBenefit
Deferred Tax Liabilities, Net   (27,000)us-gaap_DeferredTaxLiabilities  
Luxembourg Reinsurance Companies [Member]      
Deferred tax assets, net of valuation allowance      
Deferred Tax Assets, Other 183,400,000us-gaap_DeferredTaxAssetsOther
/ dei_LegalEntityAxis
= cown_LuxembourgReinsuranceCompaniesMember		    
Valuation allowance (183,400,000)us-gaap_DeferredTaxAssetsValuationAllowance
/ dei_LegalEntityAxis
= cown_LuxembourgReinsuranceCompaniesMember		    
Deferred tax liabilities      
Valuation Allowance, Amount 183,400,000us-gaap_DeferredTaxAssetsValuationAllowance
/ dei_LegalEntityAxis
= cown_LuxembourgReinsuranceCompaniesMember		    
Deferred Tax Liabilities, Gross, Current $ 204,200,000us-gaap_DeferredTaxLiabilitiesGrossCurrent
/ dei_LegalEntityAxis
= cown_LuxembourgReinsuranceCompaniesMember