| **Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Commercial 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) |
| CNA Commercial [Member]                                                                                                                                                                                               |           |           |   |   |
| Commercial Auto                                                                                                                                                                                                       | 2              | 2              | (3) | 2 |
| General Liability                                                                                                                                                                                                     | 15             | (13)           | (6) | (5) |
| Workers' Compensation                                                                                                                                                                                                 | 45             | 8              | 70 | (11) |
| Property and Other                                                                                                                                                                                                    | (35)           | (10)           | (45) | (13) |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group                                                                            | $ 27           | $ (13)         | $ 16 | $ (27) |

| X                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto**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, General Liability
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentGeneralLiability**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 and Other
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyAndOther**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, Workers' Compensation
+ References
No definition available.
+ Details**Name:**cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation**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 |