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.

+ Details

Name:	srt_CondensedFinancialStatementsCaptionsLineItems
Namespace Prefix:	srt_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

Name:	us-gaap_GuaranteeObligationsCurrentCarryingValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=115931803&loc=d3e12069-110248

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_ParentCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_OwnershipAxis=cna_CNAFConsolidatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=cna_LoewsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_IndemnificationGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Schedule II. Condensed Financial Information of Registrant (Parent Company) (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Parent Company
Condensed Financial Statements, Captions [Line Items]
Guarantor obligations, current carrying calue	$ 5	$ 5
Guarantor obligations, maximum exposure, undiscounted	$ 1,800
CNAF Consolidated \| Loews
Condensed Financial Statements, Captions [Line Items]
Noncontrolling interest, ownership percentage by parent	89.00%
Indemnification Agreement
Condensed Financial Statements, Captions [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 252
Indemnification Agreement \| Parent Company
Condensed Financial Statements, Captions [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 625