| **Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions**                             | 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 | $ (132)        | $ (175)        | $ (282) | $ (187) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (5)            | (5)            | (27) | (11) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (137)          | (180)          | (309) | (198) |
| Specialty [Member]                                                                                                                         |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (112)          | (130)          | (211) | (141) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 0              | (2)            | (18) | (10) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (112)          | (132)          | (229) | (151) |
| Commercial [Member]                                                                                                                        |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (5)            | (11)           | (37) | 0 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (3)            | (5)            | (7) | (17) |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (8)            | (16)           | (44) | (17) |
| International [Member]                                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                                             |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (15)           | (34)           | (34) | (46) |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | (2)            | 2              | (2) | 16 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | (17)           | (32)           | (36) | (30) |
| Corporate and Other Non-Core [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              | 0 | 0 |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                                             | 0              | 0              | 0 | 0 |
| Total pretax (favorable) unfavorable net prior year development, excluding Life & Group                                                | $ 0            | $ 0            | $ 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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| X                                                                                                                                                  |
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| X                                                                                                                                                     |
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| X                                                                                                                                                             |
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