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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 | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Medical Professional Liability | $ 0 | $ (2) | $ 0 | $ (2)
Other Professional Liability and Management Liability | 18 | 17 | 18 | 17
Surety | (22) | (8) | (22) | (26)
Warranty | 0 | 0 | 10 | 13
Other | 4 | (10) | 4 | (10)
Total pretax (favorable) unfavorable development | $ 0 | $ (3) | $ 10 | $ (8)

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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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Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
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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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Namespace Prefix: | cna_
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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Period Type: | duration

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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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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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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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