| **Future Policy Benefit Reserves - Narrative (Details) - USD ($)**                                    | 3 Months Ended  | 6 Months Ended  |
|-------------------------------------------------------------------------------------------------------|-----------------|-----------------|
||
| **Insurance [Abstract]**                                                                              |            |            |   |   |
| Insurance claims and policyholders benefits, including policyholders' dividends, remeasurement impact | $ (33,000,000)  | $ 1,000,000     | $ (34,000,000) | $ 6,000,000 |
| Liability for future policy benefit, expected future gross premium, discounted, before reinsurance    | $ 3,853,000,000 | $ 4,204,000,000 | $ 3,853,000,000 | $ 4,204,000,000 |
| Liability for future policy benefit, weighted-average duration                                        | 12 years        | 12 years        | 12 years | 12 years |
| Liability for future policy benefit, adverse development, expense                                     | $ 29,000,000    | $ 1,000,000     | $ 42,000,000 | $ 1,000,000 |
| Liability future policy benefit, losses recognized in prior period                                    | $ 1,000,000     | $ 0             | $ 11,000,000 | $ 1,000,000 |

| X                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Insurance Claims And Policyholders Benefits, Including Policyholders' Dividends, Remeasurement Impact
+ References
No definition available.
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| 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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense for adverse development on liability of future policy benefit from net premium exceeding gross premium.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 65 -Paragraph 2 -Subparagraph (g)(2)(iii) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480016/944-40-65-2Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-6
+ Details**Name:**us-gaap_LiabilityForFuturePolicyBenefitAdverseDevelopmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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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