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v3.22.1

Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2022 | Mar. 31, 2021
Segment Reporting Information [Line Items] |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | $ (12) | $ (15)
Specialty |  |
Segment Reporting Information [Line Items] |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | (10) | (15)
Commercial |  |
Segment Reporting Information [Line Items] |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | (2) | 0
International |  |
Segment Reporting Information [Line Items] |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | 0 | 0
Corporate & Other |  |
Segment Reporting Information [Line Items] |  |
Pretax (favorable) unfavorable premium development, excluding Life & Group | $ 0 | $ 0

X
- Definition Pretax Favorable Unfavorable Premium 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

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

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

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

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