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Claim and Claim Adjustment Expense Reserves (International - Net prior year claim and allocated claim adjustment expense reserve development) (Details) - International - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Commercial | $ (12) | $ (18) | $ (10)
Specialty | 6 | 35 | (4)
Other | 0 | (4) | 1
Total pretax (favorable) unfavorable development | $ (6) | $ 13 | $ (13)

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Specialty + References No definition available. + Details
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- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Other Professional Liability + 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 Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group + References No definition available. + Details
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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
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Balance Type: | na
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- Details
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Period Type: |