| **Future Policy Benefits Reserves - Narrative (Details) - USD ($) $ in Millions**                  | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Insurance [Abstract]**                                                                           |           |           |   |   |
| Liability for future policy benefit, expected future gross premium, discounted, before reinsurance | $ 3,546        | $ 3,853        | $ 3,546 | $ 3,853 |
| Liability for future policy benefit, weighted-average duration                                     | 11 years       | 12 years       | 11 years | 12 years |
| Liability future policy benefit, losses recognized in current period                               | $ 24           | $ 29           | $ 44 | $ 42 |
| Liability future policy benefit, losses recognized in prior period                                 | $ 6            | $ 1            | $ 8 | $ 11 |

| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability For Future Policy Benefit, Losses Recognized In Current Period Reversed Due To Favorable Development
+ References
No definition available.
+ Details**Name:**cna_LiabilityForFuturePolicyBenefitLossesRecognizedInCurrentPeriodReversedDueToFavorableDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Liability For Future Policy Benefit, Losses Recognized In Prior Period Reversed Due To Favorable Development
+ References
No definition available.
+ Details**Name:**cna_LiabilityForFuturePolicyBenefitLossesRecognizedInPriorPeriodReversedDueToFavorableDevelopment**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_InsuranceAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, before effect of reinsurance, of discounted balance for expected future gross premium component of liability for future policy benefit.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-6
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted-average duration of liability for future policy benefit, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (b)(5) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-6
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