| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (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) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (8)            | (23)           | (21) | (48) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (13)           | (75)           | (52) | (118) |
| CNA Specialty [Member]                                                                                                                     |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (41)           | (35)           | (56) | (41) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | (5)            | (13) | (14) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (46)           | (40)           | (69) | (55) |
| CNA Commercial [Member]                                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 27             | (13)           | 16 | (27) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | (19)           | (15) | (36) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | 22             | (32)           | 1 | (63) |
| Hardy [Member]                                                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 12             |           | 11 |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 2              |           | 6 |   |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | 14             |           | 17 |   |
| Corporate and Other [Member]                                                                                                               |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (3)            | (4)            | (2) | (2) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             |      | 1              | 1 | 2 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ (3)          | $ (3)          | $ (1) |    |

| 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 |