| **Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                                                           |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                       | $ (7)cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup                                                               | $ 25cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup |
| Specialty [Member]                                                                                                                                                                                               |                                                                                                                                                                                     |   |
| Medical Professional Liability                                                                                                                                                                                   | 14cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentMedicalProfessionalLiability/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember                      | 0cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentMedicalProfessionalLiability/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember |
| Other Professional Liability and Management Liability                                                                                                                                                            | (3)cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember | (6)cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember |
| Surety                                                                                                                                                                                                           | 1cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember                                             | 1cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember |
| Warranty                                                                                                                                                                                                         | 0cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember                                           | 0cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember |
| Other                                                                                                                                                                                                            | (10)cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentOther/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember                                           | 2cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentOther/ us-gaap_StatementBusinessSegmentsAxis= cna_SpecialtyMember |
| 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 |

| X                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentMedicalProfessionalLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Other
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentOther**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability
+ References
No definition available.
+ Details**Name:**cna_NetprioryearclaimandallocatedclaimadjustmentexpensereservedevelopmentOtherProfessionalLiabilityandManagementLiability**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Surety
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Warranty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtyMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |