| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 12 Months Ended                                                                                                                                                                                  |
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| **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 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Pretax Favorable Unfavorable Net Prior Year Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
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