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Fair Value (Narrative) (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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Fair Value Disclosures [Abstract] |  |
Other invested assets overseas deposit | $ 65 | $ 71

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- Definition Overseas deposits are excluded from the fair value hierarchy because their fair value is recorded using the net asset value per share (or equivalent) practical experiment, which can be redeemed at net asset value in 90 days or less. + References No definition available. + Details
Name: | cna_OtherInvestedAssetsOverseasDeposit
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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