| **Commitments, Contingencies, and Guarantees (Narrative) (Detail) (USD $)In Millions, unless otherwise specified**                 | Sep. 30, 2013 | Dec. 31, 2012 |
|------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Commitments and Contingencies Disclosure [Abstract]**                                                                            |          |          |
| Maximum potential future lease payments and other related costs under guarantee on a real estate joint venture                     | $ 90          |          |
| Aggregate amount of quantifiable indemnification agreements in effect for sales of business entities, assets and third party loans | 723           |          |
| Recorded liabilities related to indemnification agreements                                                                         | $ 7           | $ 7           |

| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum potential future lease payments and other related costs that the Company could be required to pay under the guarantee.
+ References
No definition available.
+ Details**Name:**cna_MaximumPotentialFutureLeasePaymentsAndOtherRelatedCostsUnderGuarantee**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded liabilities related to indemnification agreements
+ References
No definition available.
+ Details**Name:**cna_RecordedLiabilitiesRelatedToIndemnificationAgreements**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CommitmentsAndContingenciesDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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 FASB Interpretation (FIN) -Number 45 -Paragraph 13 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 2: 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 |