| **Legal Proceedings and Contingent Liabilities**            | 3 Months Ended                                                                                                                                                                                                                                                |
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| **Legal Proceedings and Contingent Liabilities [Abstract]** |                                                                                                                                                                                                                                                          |
| Legal Proceedings and Contingent Liabilities                | Note F. Legal Proceedings and Contingent LiabilitiesThe Company is a party to routine litigation incidental to its business, which, based on the facts and circumstances currently known, is not material to the Condensed Consolidated Financial Statements. |

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| - Definition
Legal Proceedings and Contingent Liabilities [Abstract]
+ References
No definition available.
+ Details**Name:**cna_LegalProceedingsAndContingentLiabilitiesAbstract**Namespace Prefix:**cna_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
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| - Definition
The entire disclosure for legal proceedings, legal contingencies, litigation, regulatory and environmental matters and other contingencies.
+ References
No definition available.
+ Details**Name:**us-gaap_LegalMattersAndContingenciesTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |