| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 3 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 | $ (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 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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 |
| X                                                                                                                                                                        |
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| X                                                                                                                                                  |
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| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_InternationalMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
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| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |