| **Fair Value (Assets and liabilities measured at fair value on a recurring basis) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                  | Dec. 31, 2014                                                                                                                                                                                                                                  |
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| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | $ 42,372us-gaap_AssetsFairValueDisclosureRecurring                                                                                                                                                                                             | $ 42,757us-gaap_AssetsFairValueDisclosureRecurring                                                                                                                                                                                             |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 5us-gaap_LiabilitiesFairValueDisclosureRecurring                                                                                                                                                                                               | 3us-gaap_LiabilitiesFairValueDisclosureRecurring                                                                                                                                                                                               |
| Corporate and other bonds [Member]                                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 18,552us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                                               | 18,889us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                                               |
| States, municipalities and political subdivisions [Member]                                                                                   |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 12,963us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                                      | 12,740us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                                      |
| Residential mortgage-backed [Member]                                                                                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 5,226us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                                                    | 5,233us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                                                    |
| Commercial mortgage-backed [Member]                                                                                                          |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 2,260us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                                     | 2,144us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                                     |
| Other asset-backed [Member]                                                                                                                  |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1,123us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                                                    | 1,235us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                                                    |
| Total asset-backed [Member]                                                                                                                  |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 8,609us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                                                                                                                  | 8,612us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                                                                                                                  |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                                   |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 30us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                                                   | 31us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                                                   |
| Foreign Government Debt Securities [Member]                                                                                                  |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 409us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                                          | 454us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                                          |
| Redeemable preferred stock [Member]                                                                                                          |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 42us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                                                                                                                  | 42us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                                                                                                                  |
| Fixed maturity securities [Member]                                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 40,605us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                                                 | 40,768us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                                                 |
| Equity securities [Member]                                                                                                                   |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 225us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                                                                                                         | 222us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                                                                                                         |
| Derivative Financial Instruments, Assets [Member]                                                                                            |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 2us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_DerivativeFinancialInstrumentsAssetsMember                                                                                                       |                                                                                                                                                                                                                                           |
| Other invested assets, excluding derivative financial instruments [Member]                                                                   |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 41us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                                                                                                           | 41us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                                                                                                           |
| Short term investments [Member]                                                                                                              |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1,420us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                   | 1,644us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                   |
| Life settlement contracts, included in Other assets [Member]                                                                                 |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 79us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                                                                  | 82us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                                                                  |
| Embedded Derivative Financial Instruments [Member]                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 5us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember                                                                                                | 3us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember                                                                                                |
| Level 1 [Member]                                                                                                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1,016us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                           | 957us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                             |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                          | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                          |
| Level 1 [Member] | Corporate and other bonds [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 29us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                              | 32us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                              |
| Level 1 [Member] | States, municipalities and political subdivisions [Member]                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                      | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                      |
| Level 1 [Member] | Residential mortgage-backed [Member]                                                                                      |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                   | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                   |
| Level 1 [Member] | Commercial mortgage-backed [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                    | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                    |
| Level 1 [Member] | Other asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember   | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember   |
| Level 1 [Member] | Total asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                                 | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                                 |
| Level 1 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 29us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                              | 28us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                              |
| Level 1 [Member] | Foreign Government Debt Securities [Member]                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 37us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                      | 41us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                      |
| Level 1 [Member] | Redeemable preferred stock [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 30us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                             | 30us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                             |
| Level 1 [Member] | Fixed maturity securities [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 125us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                               | 131us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                               |
| Level 1 [Member] | Equity securities [Member]                                                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 149us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                    | 145us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                    |
| Level 1 [Member] | Derivative Financial Instruments, Assets [Member]                                                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_DerivativeFinancialInstrumentsAssetsMember                  |                                                                                                                                                                                                                                           |
| Level 1 [Member] | Other invested assets, excluding derivative financial instruments [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                       | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                       |
| Level 1 [Member] | Short term investments [Member]                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 742us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                | 681us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                |
| Level 1 [Member] | Life settlement contracts, included in Other assets [Member]                                                              |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember              | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember              |
| Level 1 [Member] | Embedded Derivative Financial Instruments [Member]                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember           | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember           |
| Level 2 [Member]                                                                                                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 40,143us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                          | 40,519us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                          |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 5us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                          | 3us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                          |
| Level 2 [Member] | Corporate and other bonds [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 18,337us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                          | 18,695us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                          |
| Level 2 [Member] | States, municipalities and political subdivisions [Member]                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 12,877us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 | 12,646us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 |
| Level 2 [Member] | Residential mortgage-backed [Member]                                                                                      |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 4,994us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember               | 5,044us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember               |
| Level 2 [Member] | Commercial mortgage-backed [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 2,196us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                | 2,061us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                |
| Level 2 [Member] | Other asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 570us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember | 580us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Level 2 [Member] | Total asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 7,760us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                             | 7,685us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                             |
| Level 2 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                               | 3us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                               |
| Level 2 [Member] | Foreign Government Debt Securities [Member]                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 372us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                     | 413us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                     |
| Level 2 [Member] | Redeemable preferred stock [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 12us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                             | 12us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                             |
| Level 2 [Member] | Fixed maturity securities [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 39,359us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                            | 39,454us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                            |
| Level 2 [Member] | Equity securities [Member]                                                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 63us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                     | 61us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                     |
| Level 2 [Member] | Derivative Financial Instruments, Assets [Member]                                                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 2us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_DerivativeFinancialInstrumentsAssetsMember                  |                                                                                                                                                                                                                                           |
| Level 2 [Member] | Other invested assets, excluding derivative financial instruments [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 41us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                      | 41us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                      |
| Level 2 [Member] | Short term investments [Member]                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 678us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                | 963us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                |
| Level 2 [Member] | Life settlement contracts, included in Other assets [Member]                                                              |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember              | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember              |
| Level 2 [Member] | Embedded Derivative Financial Instruments [Member]                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 5us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember           | 3us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember           |
| Level 3 [Member]                                                                                                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1,213us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                           | 1,281us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                           |
| Fair value of liabilities, measured on a recurring basis                                                                                     | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                          | 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                          |
| Level 3 [Member] | Corporate and other bonds [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 186us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                             | 162us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CorporateDebtSecuritiesMember                             |
| Level 3 [Member] | States, municipalities and political subdivisions [Member]                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 86us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                     | 94us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                     |
| Level 3 [Member] | Residential mortgage-backed [Member]                                                                                      |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 232us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                 | 189us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                 |
| Level 3 [Member] | Commercial mortgage-backed [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 64us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                   | 83us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                   |
| Level 3 [Member] | Other asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 553us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember | 655us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Level 3 [Member] | Total asset-backed [Member]                                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 849us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                               | 927us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_AssetBackedSecuritiesMember                               |
| Level 3 [Member] | U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                               | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_USTreasuryAndGovernmentMember                               |
| Level 3 [Member] | Foreign Government Debt Securities [Member]                                                                               |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                       | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                       |
| Level 3 [Member] | Redeemable preferred stock [Member]                                                                                       |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                              | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_RedeemablePreferredStockMember                              |
| Level 3 [Member] | Fixed maturity securities [Member]                                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 1,121us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                             | 1,183us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_FixedIncomeSecuritiesMember                             |
| Level 3 [Member] | Equity securities [Member]                                                                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 13us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                     | 16us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EquitySecuritiesMember                                     |
| Level 3 [Member] | Derivative Financial Instruments, Assets [Member]                                                                         |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_DerivativeFinancialInstrumentsAssetsMember                  |                                                                                                                                                                                                                                           |
| Level 3 [Member] | Other invested assets, excluding derivative financial instruments [Member]                                                |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          |                                                                                                                                                                                                                                           | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_OtherInvestedAssetsMember                                       |
| Level 3 [Member] | Short term investments [Member]                                                                                           |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                  | 0us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_ShortTermInvestmentsMember                                  |
| Level 3 [Member] | Life settlement contracts, included in Other assets [Member]                                                              |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of assets, measured on a recurring basis                                                                                          | 79us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember             | 82us-gaap_AssetsFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember             |
| Level 3 [Member] | Embedded Derivative Financial Instruments [Member]                                                                        |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                             |                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                           |
| Fair value of liabilities, measured on a recurring basis                                                                                     | $ 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember         | $ 0us-gaap_LiabilitiesFairValueDisclosureRecurring/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_FairValueByMeasurementBasisAxis= us-gaap_EmbeddedDerivativeFinancialInstrumentsMember         |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
No definition available.
+ Details**Name:**us-gaap_AssetsFairValueDisclosureRecurring**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.
+ References
No definition available.
+ Details**Name:**us-gaap_LiabilitiesFairValueDisclosureRecurring**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=cna_LifeSettlementContractsIncludedInOtherAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=cna_OtherInvestedAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_AssetBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_DerivativeFinancialInstrumentsAssetsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EmbeddedDerivativeFinancialInstrumentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_FixedIncomeSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_RedeemablePreferredStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_ShortTermInvestmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_USTreasuryAndGovernmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |