| **Claim and Claim Adjustment Expense Reserves - Net prior year development (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | $ (55)         | $ (98)         | $ (112) | $ (150) |
| Pretax (favorable) unfavorable premium development                                                             | (8)            | (8)            | 17 | (22) |
| Total pretax (favorable) unfavorable net prior year development                                                | (63)           | (106)          | (95) | (172) |
| Specialty                                                                                                      |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | (23)           | (65)           | (54) | (99) |
| Pretax (favorable) unfavorable premium development                                                             | (5)            | (7)            | (10) | (18) |
| Total pretax (favorable) unfavorable net prior year development                                                | (28)           | (72)           | (64) | (117) |
| Commercial                                                                                                     |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | (34)           | (18)           | (58) | (32) |
| Pretax (favorable) unfavorable premium development                                                             | 1              | (2)            | 38 | (4) |
| Total pretax (favorable) unfavorable net prior year development                                                | (33)           | (20)           | (20) | (36) |
| International                                                                                                  |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | 2              | (15)           | 0 | (19) |
| Pretax (favorable) unfavorable premium development                                                             | (4)            | 1              | (11) | 0 |
| Total pretax (favorable) unfavorable net prior year development                                                | (2)            | (14)           | (11) | (19) |
| Corporate & Other Non-Core                                                                                 |           |           |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |           |           |   |   |
| Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development | 0              | 0              | 0 | 0 |
| Pretax (favorable) unfavorable premium development                                                             | 0              | 0              | 0 | 0 |
| Total pretax (favorable) unfavorable net prior year development                                                | $ 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.
+ Details**Name:**cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentexcludingLifeGroup**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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.
+ Details**Name:**us-gaap_SegmentReportingInformationLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
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