| **Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Specialty segment) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                           | $ 24           | $ (5)          | $ 49 | $ (31) |
| CNA Specialty [Member]                                                                                                                                                                                               |           |           |   |   |
| Medical Professional Liability                                                                                                                                                                                       | 1              | (17)           | 2 | (20) |
| Other Professional Liability and Management Liability                                                                                                                                                                | (58)           | (23)           | (64) | (24) |
| Surety                                                                                                                                                                                                               |      | 1              | 1 | 2 |
| Warranty                                                                                                                                                                                                             |      |      |    |    |
| Other                                                                                                                                                                                                                | 2              | (2)            | 4 | (14) |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                           | $ (55)         | $ (41)         | $ (57) | $ (56) |

| 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 |