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Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Segment Reporting Information [Line Items]      
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group $ (21)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup $ (118)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup $ (205)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
Pretax (favorable) unfavorable premium development, excluding Life & Group (32)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup (42)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup (46)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group (53)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup (160)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup (251)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 (136)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (196)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (93)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember
Pretax (favorable) unfavorable premium development, excluding Life & Group (13)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (14)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (14)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group (149)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (210)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_SpecialtyMember		 (107)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 176cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 122cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 (25)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember
Pretax (favorable) unfavorable premium development, excluding Life & Group (20)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 (8)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 (36)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group 156cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 114cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_CommercialMember		 (61)cna_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 (59)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (38)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (74)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember
Pretax (favorable) unfavorable premium development, excluding Life & Group 2cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (21)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 3cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group (57)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (59)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= cna_InternationalMember		 (71)cna_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 (2)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 (6)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 (13)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Pretax (favorable) unfavorable premium development, excluding Life & Group (1)cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 1cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 1cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group $ (3)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ (5)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember		 $ (12)cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_CorporateAndOtherMember