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Consolidated Statements of Operations - (Parenthetical) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Income Statement [Abstract] |  |  |
Insurance claims and policyholders benefits, including policyholders' dividends, remeasurement impact | $ 125 | $ 88 | $ 214

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- Definition Insurance Claims And Policyholders Benefits, Including Policyholders' Dividends, Remeasurement Impact + References No definition available. + Details
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Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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