Name:	cna_DirectAndCededEarnedPremiumsRelatedToBusinessThatIsOneHundredPercentReinsuredAsResultOfSignificantCaptiveProgram
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Largest Recoverables from Single Reinsurer

No definition available.

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Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
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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.

No definition available.

Name:	us-gaap_CededCreditRiskLineItems
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Data Type:	xbrli:stringItemType
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Amount of funds held as security for payments to be received under reinsurance contracts.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.13(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Name:	us-gaap_FundsHeldUnderReinsuranceAgreementsLiability
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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Name:	us-gaap_CededCreditRiskAxis=cna_SubsidiariesOfBerkshireHathawayGroupMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_CededCreditRiskAxis=cna_GatewayRiversInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CededCreditRiskAxis=cna_SubsidiariesofWiltonReMember
Namespace Prefix:
Data Type:	na
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