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Investments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Investments [Abstract]    
Reduction of net unrealized gains on investments included in AOCI due to shadow adjustments $ 1,370cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments $ 1,288cna_ReductionOfNetUnrealizedGainsOnInvestmentsIncludedInAociDueToShadowAdjustments
Future capital call commitments for limited partnership investments 338cna_FutureCapitalCallCommitments  
Mortgage loan commitments 41cna_MortgageLoanCommitments  
Commitments to purchase or fund privately placed debt securities 82cna_CommitmentsToPurchaseOrFundAdditionalAmounts  
Commitments to sell various privately placed debt securities $ 71cna_CommitmentsToSellVariousPrivatelyPlacedDebtSecurities