| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (205)         | $ (423) | $ (637) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (46)            | (8) | 43 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (251)           | (431) | (594) |
| CNA Specialty [Member]                                                                                                                     |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (135)           | (217) | (341) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (15)            | (28) | (3) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (150)           | (245) | (344) |
| CNA Commercial [Member]                                                                                                                    |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (46)            | (204) | (304) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (35)            | 21 | 48 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (81)            | (183) | (256) |
| Hardy [Member]                                                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (11)            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 3               |   |   |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (8)             |   |   |
| Corporate and Other [Member]                                                                                                               |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (13)            | (2) | 8 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 1               | (1) | (2) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ (12)          | $ (3) | $ 6 |

| 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 |