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Claim and Claim Adjustment Expense Reserves (Life & Group Policyholder Reserves) (Details) - USD ($) $ in Millions | 3 Months Ended
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Sep. 30, 2022 | Sep. 30, 2021
Liability for Claims and Claims Adjustment Expense [Abstract] |  |
Liability for unpaid claims and claims adjustment expense, long term care policies, period decrease | $ 25 | $ 40
Liability for unpaid claims and claims adjustment expense, incurred but not reported (IBNR) claims, amount | 107 |
Liability for unpaid claims and claims adjustment expense, claim severity adjustment, period decrease | 82 |
Liability for unpaid claims and claims adjustment expense structured settlement obligations period increase (decrease) | 5 | (2)
Liability for future policy benefits gross premium valuation pretax margin | $ 125 | $ 72

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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, Claim Severity Adjustment, Period Increase (Decrease) + References No definition available. + Details
Name: | cna_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimSeverityAdjustmentPeriodIncreaseDecrease
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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 Estimated costs as of the balance sheet date of settling insured claims and costs incurred in the claims settlement process for claims that have not yet been submitted to the insurance company for reimbursement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.W) -URI https://asc.fasb.org/extlink&oid=27015833&loc=d3e570705-122907 + Details
Name: | us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredButNotReportedIBNRClaimsAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant