| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions**                             | 3 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                                                             |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (52)         | $ (7) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (14)           | 9 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (66)           | 2 |
| Specialty [Member]                                                                                                                         |           |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (34)           | 2 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (11)           | (6) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (45)           | (4) |
| Commercial [Member]                                                                                                                        |           |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (14)           | (5) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (2)            | (1) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (16)           | (6) |
| International [Member]                                                                                                                     |           |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (4)            | (4) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (1)            | 16 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (5)            | 12 |
| 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              | 0 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 0              | 0 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 0            | $ 0 |

| 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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|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
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