| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (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 | $ (26)         | $ (18) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (13)           | (25) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (39)           | (43) |
| CNA Specialty [Member]                                                                                                                     |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (15)           | (6) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (8)            | (9) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (23)           | (15) |
| CNA Commercial [Member]                                                                                                                    |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (11)           | (14) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (10)           | (17) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (21)           | (31) |
| Hardy [Member]                                                                                                                             |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (1)            |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 4              |   |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | 3              |   |
| Corporate and Other [Member]                                                                                                               |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 1              | 2 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 1              | 1 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 2            | $ 3 |

| 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 |
| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Pretax Favorable Unfavorable Net Prior Year Development, excluding Life & Group
+ References
No definition available.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |