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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
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Mar. 31, 2024 | Mar. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items] |  |
Medical Professional Liability | $ 0 | $ 9
Other Professional Liability and Management Liability | 0 | 0
Surety | (18) | 0
Warranty | 13 | (9)
Other | 0 | 0
Total pretax (favorable) unfavorable development | $ (5) | $ 0

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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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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, Other + 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, Other Professional Liability And Management Liability + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentOtherProfessionalLiabilityAndManagementLiability
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, Surety + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentSurety
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
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty
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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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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |