| **Investments (Summary of fixed maturity and equity securities) (Details) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                    | Dec. 31, 2014                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Cost or Amortized Cost                                                                                                     | $ 37,088us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                                                          | $ 37,545us-gaap_AvailableForSaleSecuritiesAmortizedCost                                                                                                                                                                          |
| Gross Unrealized Gain                                                                                                      | 3,817us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                                                   | 3,544us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax                                                                                                                                                   |
| Gross Unrealized Loss                                                                                                      | 75us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                                                      | 99us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax                                                                                                                                                      |
| Estimated fair value                                                                                                       | 40,830us-gaap_AvailableForSaleSecurities                                                                                                                                                                                         | 40,990us-gaap_AvailableForSaleSecurities                                                                                                                                                                                         |
| Corporate and other bonds [Member]                                                                                         |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 16,721us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                    | 17,210us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                    |
| Gross Unrealized Gain                                                                                                      | 1,867us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                           | 1,721us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                           |
| Gross Unrealized Loss                                                                                                      | 43us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                              | 61us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                              |
| Estimated fair value                                                                                                       | 18,545us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                                 | 18,870us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                                 |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                             |
| States, municipalities and political subdivisions [Member]                                                                 |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 11,407us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                           | 11,285us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                           |
| Gross Unrealized Gain                                                                                                      | 1,536us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                  | 1,463us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                  |
| Gross Unrealized Loss                                                                                                      | 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                      | 8us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                      |
| Estimated fair value                                                                                                       | 12,934us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                        | 12,740us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                        |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                    | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                    |
| Residential mortgage-backed [Member]                                                                                       |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 4,998us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                         | 5,028us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                         |
| Gross Unrealized Gain                                                                                                      | 241us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                 | 218us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                 |
| Gross Unrealized Loss                                                                                                      | 13us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                  | 13us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                  |
| Estimated fair value                                                                                                       | 5,226us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                                      | 5,233us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                                      |
| Unrealized OTTI Losses (Gains)                                                                                             | (51)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember              | (53)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember              |
| Commercial mortgage-backed [Member]                                                                                        |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 2,151us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                          | 2,056us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                          |
| Gross Unrealized Gain                                                                                                      | 114us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                  | 93us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                   |
| Gross Unrealized Loss                                                                                                      | 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                    | 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                    |
| Estimated fair value                                                                                                       | 2,260us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                       | 2,144us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                       |
| Unrealized OTTI Losses (Gains)                                                                                             | (3)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                | (2)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                |
| Other asset-backed [Member]                                                                                                |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 1,109us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                         | 1,234us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                         |
| Gross Unrealized Gain                                                                                                      | 15us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                  | 11us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                  |
| Gross Unrealized Loss                                                                                                      | 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                   | 10us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                  |
| Estimated fair value                                                                                                       | 1,123us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                                      | 1,235us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                                      |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Total asset-backed [Member]                                                                                                |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 8,258us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                       | 8,318us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                       |
| Gross Unrealized Gain                                                                                                      | 370us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                               | 322us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                               |
| Gross Unrealized Loss                                                                                                      | 19us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                | 28us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                |
| Estimated fair value                                                                                                       | 8,609us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                                    | 8,612us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                                                                                                    |
| Unrealized OTTI Losses (Gains)                                                                                             | (54)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                            | (55)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_AssetBackedSecuritiesMember                            |
| U.S. Treasury and obligations of government-sponsored enterprises [Member]                                                 |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 24us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                        | 26us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                        |
| Gross Unrealized Gain                                                                                                      | 6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                               | 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                               |
| Gross Unrealized Loss                                                                                                      | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                               | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                               |
| Estimated fair value                                                                                                       | 30us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                                     | 31us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                                                                                                     |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USTreasuryAndGovernmentMember                             |
| Foreign Government Debt Securities [Member]                                                                                |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 390us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                               | 438us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                               |
| Gross Unrealized Gain                                                                                                      | 19us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                      | 16us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                      |
| Gross Unrealized Loss                                                                                                      | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                       | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                       |
| Estimated fair value                                                                                                       | 409us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                            | 454us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                            |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                     | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                     |
| Redeemable preferred stock [Member]                                                                                        |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 39us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                                                       | 39us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                                                       |
| Gross Unrealized Gain                                                                                                      | 3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                              | 3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                              |
| Gross Unrealized Loss                                                                                                      | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                              | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                              |
| Estimated fair value                                                                                                       | 42us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                                                                    | 42us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                                                                                                    |
| Unrealized OTTI Losses (Gains)                                                                                             | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                            | 0cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_RedeemablePreferredStockMember                            |
| Total fixed maturity securities available-for-sale [Member]                                                                |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 36,839us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                      | 37,316us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                      |
| Gross Unrealized Gain                                                                                                      | 3,801us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                             | 3,530us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                             |
| Gross Unrealized Loss                                                                                                      | 71us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                | 97us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                |
| Estimated fair value                                                                                                       | 40,569us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                                   | 40,749us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                                                                                                   |
| Unrealized OTTI Losses (Gains)                                                                                             | (54)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                            | (55)cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedIncomeSecuritiesMember                            |
| Total fixed maturity securities trading [Member]                                                                           |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost, Trading Securities                                                                                 | 36us-gaap_TradingSecuritiesCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedMaturitiesMember                                                                                                                  | 19us-gaap_TradingSecuritiesCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedMaturitiesMember                                                                                                                  |
| Estimated Fair Value, Trading Securities                                                                                   | 36us-gaap_TradingSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedMaturitiesMember                                                                                                                      | 19us-gaap_TradingSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_FixedMaturitiesMember                                                                                                                      |
| Common stock [Member]                                                                                                      |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 41us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                    | 38us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                    |
| Gross Unrealized Gain                                                                                                      | 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                           | 9us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                           |
| Gross Unrealized Loss                                                                                                      | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                           | 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                           |
| Estimated fair value                                                                                                       | 50us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                                 | 47us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                                 |
| Preferred stock [Member]                                                                                                   |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 172us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                | 172us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                |
| Gross Unrealized Gain                                                                                                      | 7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                        | 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                        |
| Gross Unrealized Loss                                                                                                      | 4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                        | 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                        |
| Estimated fair value                                                                                                       | 175us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                             | 175us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                             |
| Total equity securities available-for-sale [Member]                                                                        |                                                                                                                                                                                                                             |                                                                                                                                                                                                                             |
| Cost or Amortized Cost                                                                                                     | 213us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                              | 210us-gaap_AvailableForSaleSecuritiesAmortizedCost/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                              |
| Gross Unrealized Gain                                                                                                      | 16us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                     | 14us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                     |
| Gross Unrealized Loss                                                                                                      | 4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                      | 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                      |
| Estimated fair value                                                                                                       | $ 225us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                                         | $ 222us-gaap_AvailableForSaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_EquitySecuritiesMember                                                                                                         |

| X                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The accumulated unrealized gain or loss position on Other Than Temporary Impariment (OTTI) Securities
+ References
No definition available.
+ Details**Name:**cna_AccumulatedOtherComprehensiveIncomeLossOtherThanTemporaryImpairmentNotCreditLossBeforeTaxDebtSecurities**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
+ Details**Name:**us-gaap_TradingSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cost of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.
+ References
No definition available.
+ Details**Name:**us-gaap_TradingSecuritiesCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                           |
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| X                                                                                                                                                                                                         |
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| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                         |
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| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
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