| **Legal Proceedings, Contingencies and Guarantees**                             | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Legal Proceedings, Contingencies and Guarantees                                 | Legal Proceedings, Contingencies and GuaranteesThe Company is a party to various claims and litigation incidental to its business, which, based on the facts and circumstances currently known, are not material to the Company's results of operations or financial position.Guarantees The Company has provided guarantees, if the primary obligor fails to perform, to holders of structured settlement annuities issued by a previously owned subsidiary.  As of September 30, 2023, the potential amount of future payments the Company could be required to pay under these guarantees was $1.5 billion, which will be paid over the lifetime of the annuitants.  The Company does not believe any payment is likely under these guarantees, as the Company is the beneficiary of a trust that must be maintained at a level that approximates the discounted reserves for these annuities. |

| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Legal Proceedings, Commitments and Contingencies, and Guarantees [Abstract]
+ References
No definition available.
+ Details**Name:**cna_LegalProceedingsCommitmentsandContingenciesandGuaranteesAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Legal Proceedings, Commitments and Contingencies, and Guarantees [Text Block]
+ References
No definition available.
+ Details**Name:**cna_LegalProceedingsCommitmentsandContingenciesandGuaranteesTextBlock**Namespace Prefix:**cna_**Data Type:**dtr-types1:textBlockItemType**Balance Type:**na**Period Type:**duration |