| **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 premium development, excluding Life & Group                                 | $ (181)         | $ (308) | $ (288) |
| Specialty                                                                                                      |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | (150)           | (174) | (247) |
| Commercial                                                                                                     |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | (25)            | (115) | 15 |
| International                                                                                                  |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | (4)             | (9) | (58) |
| Corporate & Other                                                                                          |            |   |   |
| **Segment Reporting Information [Line Items]**                                                                 |            |   |   |
| Pretax (favorable) unfavorable premium development, excluding Life & Group                                 | $ (2)           | $ (10) | $ 2 |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pretax Favorable Unfavorable Premium 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.
+ 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                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
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