Schedule II. Condensed Financial Information of Registrant (Parent Company) (Schedule II. Condensed Financial Information of Registrant (Narrative)) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Cumulative Effect of New Accounting Principle in Period of Adoption on Other Assets		$ (2)
Cumulative Effect of New Accounting Principle in Period of Adoption on Long Term Debt		(2)
Parent Company [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption on Other Assets		(2)
Cumulative Effect of New Accounting Principle in Period of Adoption on Long Term Debt		(2)
Guarantor Obligations, Current Carrying Value	$ 5	$ 5
Guarantor Obligations, Maximum Exposure, Undiscounted	1,900
Indemnification Agreement [Member]
Guarantor Obligations, Maximum Exposure, Undiscounted	258
Indemnification Agreement [Member] \| Parent Company [Member]
Guarantor Obligations, Maximum Exposure, Undiscounted	$ 625

Amount after tax of decrease (increase) to long term debt, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

No definition available.

+ Details

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

- Definition

Amount after tax of increase (decrease) to other assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

+ References

No definition available.

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6851643&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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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

- Details

Name:	dei_LegalEntityAxis=us-gaap_ParentCompanyMember
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: