| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $)In Millions, unless otherwise specified**      | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Segment Reporting Information [Line Items]**                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (118)         | $ (205) | $ (423) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (42)            | (46) | (8) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (160)           | (251) | (431) |
| 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 | (230)           | (135) | (217) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (17)            | (15) | (28) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (247)           | (150) | (245) |
| 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 | 104             | (46) | (204) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (9)             | (35) | 21 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | 95              | (81) | (183) |
| Hardy [Member]                                                                                                                             |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 14              | (11) |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (17)            | 3 |   |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (3)             | (8) |   |
| 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 | (6)             | (13) | (2) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 1               | 1 | (1) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ (5)           | $ (12) | $ (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:**debit**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:**debit**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:**debit**Period Type:**duration |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |