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Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Segment Reporting Information [Line Items] |  |
Total pretax (favorable) unfavorable development | $ (7) | $ 13
Specialty |  |
Segment Reporting Information [Line Items] |  |
Total pretax (favorable) unfavorable development | (5) | 0
Commercial |  |
Segment Reporting Information [Line Items] |  |
Total pretax (favorable) unfavorable development | (2) | (2)
International |  |
Segment Reporting Information [Line Items] |  |
Total pretax (favorable) unfavorable development | 0 | 15
Corporate & Other |  |
Segment Reporting Information [Line Items] |  |
Total pretax (favorable) unfavorable development | $ 0 | $ 0

X
- Definition Pretax Favorable Unfavorable Premium Development, excluding Life & Group + References No definition available. + Details
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Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=cna_CommercialSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=cna_InternationalSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateAndOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |