| **Legal Proceedings, Contingencies and Guarantees (Narrative) (Details) - USD ($) $ in Millions** | Dec. 31, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------------|---------------|---------------|
| Guarantee Obligations                                                                             |          |          |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |          |          |
| Guarantor obligations, maximum exposure, undiscounted                                             | $ 1,700       |          |
| Guarantee and Indemnification                                                                     |          |          |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**                 |          |          |
| Guarantor obligations, current carrying value                                                     | $ 5           | $ 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
The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248
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| 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=118943654&loc=d3e12069-110248
+ Details**Name:**us-gaap_GuaranteeObligationsMaximumExposure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                   |
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| X                                                                                                                                                                                |
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