| **Schedule of Other Intangible Assets (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended |        |
|--------------------------------------------------------------------------------------------------|----------------|--------|
||
| Indefinite-lived intangible assets                                                               | $ 74           | $ 74   |
| Finite-Lived Intangible Assets                                                                   | 86             | 85     |
| Accumulated Amortization                                                                         | 67             | 66     |
| Total other intangible assets                                                                    | 160            | 159    |
| Total intangible assets, accumulated amortization                                                | 67             | 66     |
| Lloyds syndicate capacity [Member]                                                               |           |   |
| Indefinite-lived intangible assets                                                               | 58             | 58     |
| Agency Force [Member]                                                                            |           |   |
| Indefinite-lived intangible assets                                                               | 16             | 16     |
| Value of business acquired [Member]                                                              |           |   |
| Finite-Lived Intangible Assets                                                                   | 64             | 64     |
| Accumulated Amortization                                                                         | 64             | 63     |
| Trade name [Member]                                                                              |           |   |
| Economic useful life measured in years                                                           | 8 years        |   |
| Finite-Lived Intangible Assets                                                                   | 9              | 8      |
| Accumulated Amortization                                                                         | 2              | 2      |
| Distribution channel [Member]                                                                    |           |   |
| Economic useful life measured in years                                                           | 15 years       |   |
| Finite-Lived Intangible Assets                                                                   | 13             | 13     |
| Accumulated Amortization                                                                         | $ 1            | $ 1    |
| Minimum [Member] | Value of business acquired [Member]                                           |           |   |
| Economic useful life measured in years                                                           | 1 year         |   |
| Maximum [Member] | Value of business acquired [Member]                                           |           |   |
| Economic useful life measured in years                                                           | 4 years        |   |

| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Intangible Assets
+ References
No definition available.
+ Details**Name:**cna_OtherIntangibleAssets**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total intangible assets, accumulated amortization
+ References
No definition available.
+ Details**Name:**cna_TotalIntangibleAssetsAccumulatedAmortization**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
+ Details**Name:**us-gaap_FiniteLivedIntangibleAssetsGross**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
No definition available.
+ Details**Name:**us-gaap_FiniteLivedIntangibleAssetUsefulLife**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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