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Income Taxes (Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Federal income taxes received from (paid to) Loews $ (287,000,000)cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount $ (89,000,000)cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount $ 75,000,000cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount
Unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits  
Interest income (expense) 0us-gaap_InterestIncomeExpenseNet 0us-gaap_InterestIncomeExpenseNet 2,000,000us-gaap_InterestIncomeExpenseNet
Unrecognized Tax Benefits, Income Tax Penalties Expense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense
Accrued interest and penalties 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued  
Deferred tax liabilities not recognized on undistributed earnings 0us-gaap_DeferredTaxLiabilityNotRecognizedAmountOfUnrecognizedDeferredTaxLiabilityUndistributedEarningsOfForeignSubsidiaries    
Foreign tax expense (benefit) on income from continuing operations 24,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations 24,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations 34,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations
Income (loss) from continuing foreign operations 66,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign 101,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign 88,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign
Tax Credit Carryforward, Amount 0us-gaap_TaxCreditCarryforwardAmount    
Valuation allowance $ 0us-gaap_DeferredTaxAssetsValuationAllowance $ 0us-gaap_DeferredTaxAssetsValuationAllowance