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Claim and Claim Adjustment Expense Reserves (Specialty - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - Specialty - 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] |  |  |
Medical Professional Liability | $ (8) | $ 5 | $ 18
Other Professional Liability and Management Liability | 49 | 37 | 50
Surety | (68) | (43) | (83)
Warranty | 20 | (11) | (21)
Other | (2) | (2) | (4)
Total pretax (favorable) unfavorable development | $ (9) | $ (14) | $ (40)

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Medical Professional Liability + 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, Other + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Surety + 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, Warranty + References No definition available. + Details
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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management 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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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
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Balance Type: | na
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- Details
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