| **Legal Proceedings and Guarantees - Narrative (Details) - USD ($) $ in Millions** | 3 Months Ended | 6 Months Ended |        |
|------------------------------------------------------------------------------------|----------------|----------------|--------|
||
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Adverse premium development recognized as result of rating errors                  | $ 1            | $ 37           |   |
| Guarantee Obligations                                                              |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Guarantor obligations, maximum exposure, undiscounted                              | 1,800          | 1,800          |   |
| Indemnification Agreement                                                          |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Guarantor obligations, maximum exposure, undiscounted                              | 254            | 254            |   |
| Guarantee agreement                                                                |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Guarantor obligations, maximum exposure, undiscounted                              | 434            | 434            |   |
| Guarantee and Indemnification                                                      |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Guarantor obligations, current carrying value                                      | 5              | 5              | $ 5    |
| Insurance Premium Rate Adjustment                                                  |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Interest due to policyholders                                                      | 1              | 6              |   |
| Multi-Peril Package Product | Insurance Premium Rate Adjustment                    |           |           |   |
| **Legal Proceedings, Commitments and Contingencies, and Guarantees [Line Items]**  |           |           |   |
| Loss contingency accrual                                                           | $ 96           | $ 96           |   |

| X                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adverse Premium Development Due To The Small Business Premium Rate Adjustment Contingency, Recognized
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82913245&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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of loss contingency liability.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount charged against operating income increasing loss contingency liability, after adjustments to reduce previously estimated charges.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349
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| X                                                                                                                                                                   |
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| X                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
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