| **Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) $ in Billions** | Sep. 30, 2023 USD ($) |
|-----------------------------------------------------------------------------------------|-----------------------|
| Guarantee Obligations                                                                   |                  |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**       |                  |
| Guarantor obligations, maximum exposure, undiscounted                                   | $ 1.5                 |

| X                                                                                                                                                                                                                                                                                                                                          |
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| - Definition
[Line Items] for Legal Proceedings, Commitments and Contingencies, and Guarantees [Table]
+ References
No definition available.
+ Details**Name:**cna_LegalProceedingsCommitmentsandContingenciesandGuaranteesLineItems**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 460 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482425/460-10-50-4
+ Details**Name:**us-gaap_GuaranteeObligationsMaximumExposure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                   |
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| - Details**Name:**us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_GuaranteeObligationsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |