| **Fair Value (Quantitative information about significant unobservable inputs in the fair value measurement of level 3 assets) (Details) (USD $)In Millions, except Per Share data, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                               | Dec. 31, 2014                                                                                                                                                                                                                                               |
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| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member]                                                                                                                               |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 102us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember                                                              | $ 101us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember                                                              |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Credit Spread                                                                                                                                                                                                   | 2.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember            | 2.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember            |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Credit Spread                                                                                                                                                                                                   | 13.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember           | 13.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember           |
| Fixed maturity securities [Member] | Income Approach Valuation Technique [Member] | Weighted Average [Member]                                                                                                   |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Credit Spread                                                                                                                                                                                                   | 3.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember    | 3.00%cna_FairValueInputsCreditRiskCounterparty/ us-gaap_FairValueByAssetClassAxis= us-gaap_FixedIncomeSecuritiesMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember    |
| Equity securities [Member] | Market Approach Valuation Technique [Member]                                                                                                                                       |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | 13us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                                                      | 16us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                                                      |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Minimum [Member]                                                                                                                    |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Private offering price                                                                                                                                                                                          | $ 10cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                   | $ 12cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                   |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Maximum [Member]                                                                                                                    |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Private offering price                                                                                                                                                                                          | $ 4,400cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                | $ 4,391cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                                |
| Equity securities [Member] | Market Approach Valuation Technique [Member] | Weighted Average [Member]                                                                                                           |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Private offering price                                                                                                                                                                                          | $ 682cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                          | $ 600cna_PrivateOfferingPrice/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_MarketApproachValuationTechniqueMember                          |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member]                                                                                                     |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Assets, Fair Value Disclosure                                                                                                                                                                                   | $ 79us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember                                            | $ 82us-gaap_AssetsFairValueDisclosure/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember                                            |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Minimum [Member]                                                                                  |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Mortality assumption                                                                                                                                                                                            | 55.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember          | 55.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember          |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Maximum [Member]                                                                                  |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Mortality assumption                                                                                                                                                                                            | 1676.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember        | 1676.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember        |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Weighted Average [Member]                                                                         |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Mortality assumption                                                                                                                                                                                            | 164.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember | 163.00%cna_MortalityAssumption/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= us-gaap_WeightedAverageMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember |
| Life settlement contracts, included in Other assets [Member] | Income Approach Valuation Technique [Member] | Rate [Member]                                                                                     |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| **Fair Value Inputs, Assets, Quantitative Information [Line Items]**                                                                                                                                            |                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                        |
| Discount rate risk premium                                                                                                                                                                                      | 9.00%cna_DiscountRateRiskPremium/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= cna_RateMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember              | 9.00%cna_DiscountRateRiskPremium/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember/ us-gaap_RangeAxis= cna_RateMember/ us-gaap_ValuationTechniqueAxis= us-gaap_IncomeApproachValuationTechniqueMember              |

| X                                                                                                                                                                                                                               |
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| - Definition
Discount rate risk premium
+ References
No definition available.
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| X                                                                                                                                                                                                                                                               |
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| - Definition
Fair Value Inputs, Credit Risk, Counterparty
+ References
No definition available.
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| X                                                                                                                                                                                                                     |
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| - Definition
Mortality assumption
+ References
No definition available.
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| - Definition
Private offering price
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                |
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| X                                                                                                                                                        |
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| X                                                                                                                                                             |
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