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Reinsurance (Voluntary Reinsurance) (Details) $ in Millions | Dec. 31, 2021 USD ($)
--- | ---
Effects of Reinsurance [Line Items] |
Total voluntary reinsurance outstanding balance | $ 4,802
A- to A++ |
Effects of Reinsurance [Line Items] |
Total voluntary reinsurance outstanding balance | 3,812
B- to B++ |
Effects of Reinsurance [Line Items] |
Total voluntary reinsurance outstanding balance | 987
Insolvent |
Effects of Reinsurance [Line Items] |
Total voluntary reinsurance outstanding balance | $ 3

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- Definition Voluntary Reinsurance Recoverable, Gross + References No definition available. + Details
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Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
Name: | us-gaap_EffectsOfReinsuranceLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | cna_CreditRatingAxis=cna_AMinusToAPlusPlusMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cna_CreditRatingAxis=cna_BMinusToBPlusPlusMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | cna_CreditRatingAxis=cna_InsolventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |