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v3.25.0.1

Claim and Claim Adjustment Expense Reserves (Net Prior Year Development) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | $ 48 | $ 48 | $ (32)
Corporate & Other |  |  |
Segment Reporting Information [Line Items] |  |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | $ 79 | $ 71 | $ 64

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