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Benefit Plans (Narrative) (details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Lump sum settlement acceptance rate by retirement plan participants 20.00%cna_Lumpsumsettlementacceptanceratebyretirementplanparticipants    
Lump sum pension settlement payment $ 253cna_Lumpsumpensionsettlementpayment    
Settlement loss 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements    
Curtailment gain (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments    
Future capital call commitments for limited partnership investments 327cna_FutureCapitalCallCommitments    
Document Period End Date Dec. 31, 2014    
Pension Plans, Defined Benefit [Member]      
Settlement loss 84cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
3cna_DefinedBenefitPlanRecognizedNetLossDuetoSettlements
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
 
Defined benefit plan, accumulated benefit obligation 2,962us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
2,889us-gaap_DefinedBenefitPlanAccumulatedBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
 
Future capital call commitments for limited partnership investments 107cna_FutureCapitalCallCommitments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Limited partnership equity related hedge fund strategy plan assets 57.00%cna_LimitedPartnershipEquityRelatedHedgeFundStrategyPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Limited partnership hedge fund multi strategy plan assets 37.00%cna_LimitedPartnershipHedgeFundMultiStrategyPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Limited partnership hedge fund strategy distressed investments plan assets 6.00%cna_LimitedPartnershipHedgeFundStrategyDistressedInvestmentsPlanAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year 14us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember
   
Cost of Living Adjustment [Member]      
Assumptions used in calculating assumed health care cost trend rate 4.00%cna_Assumptionsusedincalculatingassumedhealthcarecosttrendrate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_CostofLivingAdjustmentMember
   
Employer Subsidy on Health Care Costs [Member]      
Assumptions used in calculating assumed health care cost trend rate 4.00%cna_Assumptionsusedincalculatingassumedhealthcarecosttrendrate
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_EmployerSubsidyonHealthCareCostsMember
   
Other Postretirement Benefit Plan [Member]      
Curtailment gain (86)cna_DefinedBenefitPlanRecognizedNetGainDuetoCurtailments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Defined benefit plan, health care cost trend rate assumed for next fiscal year 4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
4.00%us-gaap_DefinedBenefitPlanHealthCareCostTrendRateAssumedForNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year 5us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
   
Equity securities [Member]      
Defined Benefit Plan, Target Plan Asset Allocations Range Minimum 40.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_EquitySecuritiesMember
   
Defined Benefit Plan, Target Plan Asset Allocations Range Maximum 60.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_EquitySecuritiesMember
   
Defined Contribution Pension [Member]      
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 50.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
   
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 70.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
   
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
   
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years 5 years    
Company Contribution of Eligible Compensation, Depending on Age, Percent 3% or 5%    
Benefit expense for the Company's savings plan $ 69us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
71us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
70us-gaap_DefinedContributionPlanCostRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_OtherPensionPlansDefinedBenefitMember
First Year of Employment [Member]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 35.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_FirstYearofEmploymentMember
   
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_FirstYearofEmploymentMember
   
Additional Contribution, Management Discretion [Member]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 80.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_AdditionalContributionManagementDiscretionMember
   
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 6.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_AdditionalContributionManagementDiscretionMember
   
Defined Contribution Plan, Employers Matching Contribution, Vesting Period in Years 5 years    
Defined Contribution Plan, Employer Discretionary Contribution Amount, Percent 2.00%cna_DefinedContributionPlanEmployerDiscretionaryContributionAmountPercent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= cna_AdditionalContributionManagementDiscretionMember