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Reinsurance (Components of Reinsurance Receivables) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Reinsurance Disclosures [Abstract]        
Ceded claim and claim adjustment expenses $ 4,344cna_CededClaimAndClaimAdjustmentExpenses $ 4,972cna_CededClaimAndClaimAdjustmentExpenses $ 5,126cna_CededClaimAndClaimAdjustmentExpenses $ 5,020cna_CededClaimAndClaimAdjustmentExpenses
Ceded future policy benefits 185cna_CededFuturePolicyBenefits 733cna_CededFuturePolicyBenefits    
Ceded policyholders' funds   35cna_CededPolicyholdersFunds    
Reinsurance receivables related to paid losses 213us-gaap_ReinsuranceRecoverablesOnPaidLossesGross 348us-gaap_ReinsuranceRecoverablesOnPaidLossesGross    
Reinsurance receivables 4,742us-gaap_ReinsuranceRecoverablesGross 6,088us-gaap_ReinsuranceRecoverablesGross    
Allowance for uncollectible reinsurance (48)us-gaap_ReinsuranceRecoverablesAllowance (71)us-gaap_ReinsuranceRecoverablesAllowance    
Reinsurance receivables, net of allowance for uncollectible reinsurance $ 4,694us-gaap_ReinsuranceRecoverables $ 6,017us-gaap_ReinsuranceRecoverables