| **Hardy Schedule of intangible assets acquired (Details) (USD $)In Millions, unless otherwise specified** | 9 Months Ended |        |
|-----------------------------------------------------------------------------------------------------------|----------------|--------|
||
| Indefinite-lived intangible assets                                                                        |           | $ 55   |
| Finite-lived intangible assets                                                                            |           | 81     |
| Total intangible assets as of the acquisition date                                                        |           | 136    |
| Lloyds syndicate capacity [Member]                                                                        |           |   |
| Indefinite-lived intangible assets                                                                        |           | 55     |
| Value of business acquired [Member]                                                                       |           |   |
| Finite-lived intangible assets                                                                            |           | 60     |
| Trade Name [Member]                                                                                       |           |   |
| Economic useful life measured in years                                                                    | 8              |   |
| Finite-lived intangible assets                                                                            |           | 8      |
| Distribution Rights [Member]                                                                              |           |   |
| Economic useful life measured in years                                                                    | 15             |   |
| Finite-lived intangible assets                                                                            |           | $ 13   |
| Minimum [Member] | Lloyds syndicate capacity [Member]                                                     |           |   |
| Economic useful life measured in years                                                                    | 1              |   |
| Maximum [Member] | Lloyds syndicate capacity [Member]                                                     |           |   |
| Economic useful life measured in years                                                                    | 4              |   |

| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business acquisition, purchase price allocation, intangible assets
+ References
No definition available.
+ Details**Name:**cna_BusinessAcquisitionPurchasePriceAllocationIntangibleAssets**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Economic useful life measured in years
+ References
No definition available.
+ Details**Name:**cna_EconomicUsefulLifeMeasuredInYears**Namespace Prefix:**cna_**Data Type:**us-types:durationStringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of acquisition cost of a business combination allocated to an identifiable intangible asset that will be amortized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph e -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 Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 52 -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.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of acquisition cost of a business combination allocated to an identifiable intangible asset that will not be amortized. Does not include goodwill.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 51 -Subparagraph e -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 Statement of Financial Accounting Standard (FAS) -Number 141 -Paragraph 52 -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.
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