| **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 | $ (52)         | $ (111)        | $ (70) | $ (130) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (23)           | 39             | (48) | 23 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (75)           | (72)           | (118) | (107) |
| CNA Specialty [Member]                                                                                                                     |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (35)           | (52)           | (41) | (67) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | (1)            | (14) | (8) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (40)           | (53)           | (55) | (75) |
| CNA Commercial [Member]                                                                                                                    |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (13)           | (50)           | (27) | (57) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (19)           | 40             | (36) | 32 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (32)           | (10)           | (63) | (25) |
| Corporate and Other Non-Core [Member]                                                                                                      |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (4)            | (9)            | (2) | (6) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 1              |      | 2 | (1) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ (3)          | $ (9)          |    | $ (7) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C
+ 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
+ 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 |