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Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Life & Group Policyholder Reserves) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2021
Liability for Claims and Claims Adjustment Expense [Abstract] |  |  |  |
Liability for unpaid claims and claims adjustment expense, long term care policies, period increase (decrease) | $ (40) | $ (37) | $ (56) |
Liability for unpaid claims and claims adjustment expense structured settlement obligations period increase (decrease) | 2 | 46 |  |
Liability for future policy benefits, gross premium valuation, pretax margin | $ 72 |  |  | $ 72
Premium deficiency and future policy benefit |  | $ 74 | $ 216 |

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- Definition Liability for Future Policy Benefits, Gross Premium Valuation, Pretax Margin + References No definition available. + Details
Name: | cna_LiabilityForFuturePolicyBenefitsGrossPremiumValuationPretaxMargin
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Liability for Unpaid Claims and Claims Adjustment Expense, Long Term Care Policies, Period Increase (Decrease) + References No definition available. + Details
Name: | cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseLongTermCarePoliciesPeriodIncreaseDecrease
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Liability For Unpaid Claims And Claims Adjustment Expense Structured Settlement Obligations Period Increase (Decrease) + References No definition available. + Details
Name: | cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseStructuredSettlementObligationsPeriodIncreaseDecrease
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpenseAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of expense (income) recognized due to changes in the accrued obligation to policyholders that relates to insured events. + References No definition available. + Details
Name: | us-gaap_LiabilityForFuturePolicyBenefitsPeriodExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration