| **Schedule of Other Intangible Assets (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                                        |                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| Indefinite-lived intangible assets                                                               | $ 71us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill                                                                                           | $ 74us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill                                                                                           |
| Finite-Lived Intangible Assets                                                                   | 80us-gaap_FiniteLivedIntangibleAssetsGross                                                                                                             | 85us-gaap_FiniteLivedIntangibleAssetsGross                                                                                                             |
| Intangible Assets, Gross (Excluding Goodwill)                                                    | 151us-gaap_IntangibleAssetsGrossExcludingGoodwill                                                                                                      | 159us-gaap_IntangibleAssetsGrossExcludingGoodwill                                                                                                      |
| Accumulated Amortization                                                                         | 66us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                           | 66us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                           |
| Lloyds syndicate capacity [Member]                                                               |                                                                                                                                                   |                                                                                                                                                   |
| Indefinite-lived intangible assets                                                               | 55us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= cna_LloydsSyndicateCapacityMember | 58us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= cna_LloydsSyndicateCapacityMember |
| Agency Force [Member]                                                                            |                                                                                                                                                   |                                                                                                                                                   |
| Indefinite-lived intangible assets                                                               | 16us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= cna_AgencyForceMember             | 16us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis= cna_AgencyForceMember             |
| Value of business acquired [Member]                                                              |                                                                                                                                                   |                                                                                                                                                   |
| Finite-Lived Intangible Assets                                                                   | 60us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= cna_ValueOfBusinessAcquiredMember                     | 64us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= cna_ValueOfBusinessAcquiredMember                     |
| Accumulated Amortization                                                                         | 61us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= cna_ValueOfBusinessAcquiredMember   | 63us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= cna_ValueOfBusinessAcquiredMember   |
| Trade name [Member]                                                                              |                                                                                                                                                   |                                                                                                                                                   |
| Economic useful life measured in years                                                           | 8 years                                                                                                                                                |                                                                                                                                                   |
| Finite-Lived Intangible Assets                                                                   | 8us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember                               | 8us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember                               |
| Accumulated Amortization                                                                         | 3us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember             | 2us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_TradeNamesMember             |
| Distribution channel [Member]                                                                    |                                                                                                                                                   |                                                                                                                                                   |
| Economic useful life measured in years                                                           | 15 years                                                                                                                                               |                                                                                                                                                   |
| Finite-Lived Intangible Assets                                                                   | 12us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_DistributionRightsMember                      | 13us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_DistributionRightsMember                      |
| Accumulated Amortization                                                                         | $ 2us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_DistributionRightsMember   | $ 1us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_DistributionRightsMember   |
| 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
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
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| 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
+ Details**Name:**us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before accumulated amortization of intangible assets, excluding goodwill.
+ References
No definition available.
+ Details**Name:**us-gaap_IntangibleAssetsGrossExcludingGoodwill**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TradeNamesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=cna_AgencyForceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis=cna_LloydsSyndicateCapacityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |