| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 3 Months Ended | 9 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (81)         | $ (40)         | $ (112) | $ (110) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 5              | (7)            | (16) | (55) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (76)           | (47)           | (128) | (165) |
| CNA Specialty [Member]                                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (74)           | (39)           | (130) | (80) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (3)            | (1)            | (16) | (15) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (77)           | (40)           | (146) | (95) |
| CNA Commercial [Member]                                                                                                                    |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (3)            | 2              | 13 | (25) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 7              | (5)            | (8) | (41) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | 4              | (3)            | 5 | (66) |
| Hardy [Member]                                                                                                                             |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (6)            | (6)            | 5 | (6) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 1              |      | 7 |    |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (5)            | (6)            | 12 | (6) |
| Corporate and Other [Member]                                                                                                               |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 2              | 3              |    | 1 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             |      | (1)            | 1 | 1 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 2            | $ 2            | $ 1 | $ 2 |

| 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 |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |