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Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Segment Reporting Information [Line Items]    
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group $ (7)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup $ 25cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
Pretax (favorable) unfavorable premium development, excluding Life & Group 9cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup (31)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group 2cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup (6)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
Specialty [Member]    
Segment Reporting Information [Line Items]    
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group 2cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (3)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember
Pretax (favorable) unfavorable premium development, excluding Life & Group (6)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (6)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group (4)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (9)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember
Commercial [Member]    
Segment Reporting Information [Line Items]    
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group (5)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 18cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember
Pretax (favorable) unfavorable premium development, excluding Life & Group (1)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 (18)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group (6)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 0cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember
International [Member]    
Segment Reporting Information [Line Items]    
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group (4)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 10cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember
Pretax (favorable) unfavorable premium development, excluding Life & Group 16cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (7)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group 12cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 3cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember
Corporate and Other [Member]    
Segment Reporting Information [Line Items]    
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group 0cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 0cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Pretax (favorable) unfavorable premium development, excluding Life & Group 0cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 0cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group $ 0cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ 0cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember