| **Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for 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                                                                 | $ 24           | $ (5)          | $ 49 | $ (31) |
| Hardy [Member]                                                                                                                                                                                             |           |           |   |   |
| Marine and Aviation                                                                                                                                                                                        | 1              | 4              | 6 | 3 |
| Non-Marine Property                                                                                                                                                                                        | 3              | 7              | 1 | 7 |
| Property Treaty                                                                                                                                                                                            | (2)            | 2              | (4) | 2 |
| Specialty                                                                                                                                                                                                  | (2)            | (1)            | (3) | (1) |
| Commutations                                                                                                                                                                                               |      |      | 10 |    |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                 |      | $ 12           | $ 10 | $ 11 |

| X                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Commutation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentCommutation**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, Marine and Aviation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentMarineandAviation**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, Non-Marine Property
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentNonMarineProperty**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, Property Treaty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyTreaty**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, Specialty
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentSpecialty**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 |