| **Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Hardy 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                                                                       | $ (5)          | $ (52)         | $ (31) | $ (70) |
| Hardy [Member]                                                                                                                                                                                                   |           |           |   |   |
| Marine and Aviation                                                                                                                                                                                              | 4              |           | 3 |   |
| Non-Marine Property                                                                                                                                                                                              | 7              |           | 7 |   |
| Property Treaty                                                                                                                                                                                                  | 2              |           | 2 |   |
| Specialty                                                                                                                                                                                                        | (1)            |           | (1) |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                       | $ 12           |           | $ 11 |   |

| X                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Marine and Aviation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentMarineandAviation**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Non-Marine Property
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentNonMarineProperty**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Property Treaty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyTreaty**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Specialty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentSpecialty**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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:**credit**Period Type:**duration |