| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions**                             | 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 | $ (210)         | $ (21) | $ (118) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (8)             | (32) | (42) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (218)           | (53) | (160) |
| Specialty [Member]                                                                                                                         |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (141)           | (136) | (196) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (11)            | (13) | (14) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (152)           | (149) | (210) |
| Commercial [Member]                                                                                                                        |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (15)            | 176 | 122 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (15)            | (20) | (8) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (30)            | 156 | 114 |
| International [Member]                                                                                                                     |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |            |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (54)            | (59) | (38) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 18              | 2 | (21) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (36)            | (57) | (59) |
| 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 | 0               | (2) | (6) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 0               | (1) | 1 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 0             | $ (3) | $ (5) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group
+ References
No definition available.
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| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                  |
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
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