Name:	cna_Assetsheldforsale
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Equity securities

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationEquitySecurities
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Fixed maturity securities

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationFixedMaturitySecurities
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Insurance reserves

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationInsuranceReserves
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Disposal group including discontinued operation, Reinsurance receivables

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationReinsuranceReceivables
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Separate account business assets

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationSeparateAccountBusinessAssets
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Separate account business liabilities

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationSeparateAccountBusinessLiabilities
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Disposal group including discontinued operation, Short term investments

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationShortTermInvestments
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Disposal group including discontinued operation, Total investments

No definition available.

Name:	cna_DisposalGroupIncludingDiscontinuedOperationTotalInvestments
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Impairment On Assets Held For Sale

No definition available.

Name:	cna_ImpairmentOnAssetsHeldForSale
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The aggregate value (measured at the lower of net carrying value or fair value less cost of disposal) for assets of a disposal group, including a component of the entity (discontinued operation), to be sold or that has been disposed of through sale, as of the financial statement date.

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Namespace Prefix:	us-gaap_
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- Details

Name:	us-gaap_AssetsOfDisposalGroupIncludingDiscontinuedOperationAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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For the disposal group, including a component of the entity (discontinued operation), cash includes currency on hand as well as demand deposits with banks or financial institutions. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

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

For the disposal group, including a component of the entity (discontinued operation), carrying amount as of the balance sheet date of assets not otherwise specified.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationOtherAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

For the disposal group, including a component of the entity (discontinued operation), obligations not otherwise itemized.

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

The obligations arising from the sale, disposal, or planned sale in the near future (generally within one year) of a disposal group, including a component of the entity (discontinued operation).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 45

-Paragraph 10

-URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 20

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1436-107760