| **Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                           |
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| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | $ 1,281us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                          | $ 1,303us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 18us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                                                                         | 17us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss                                                                                                       | 12us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss |
| Purchases                                                                                                                                                                                                      | 125us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases                                                                                                                                          | 178us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases |
| Sales                                                                                                                                                                                                          | (156)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales                                                                                                                                            | (95)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales |
| Settlements                                                                                                                                                                                                    | (53)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements                                                                                                                                       | (110)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements |
| Transfers into Level 3                                                                                                                                                                                         | 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3                                                                                                                                 | 38us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 |
| Transfers out of Level 3                                                                                                                                                                                       | (46)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3                                                                                                                              | (55)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 |
| Balance, Ending, Assets                                                                                                                                                                                        | 1,213us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                            | 1,288us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 1cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld                                                                                                                                                          | 1cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld |
| Fixed maturity securities [Member]                                                                                                                                                                             |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 1,183us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                           | 1,203us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                         | 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 10us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                     | 16us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 125us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                         | 178us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Sales                                                                                                                                                                                                          | (156)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                           | (87)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (37)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                      | (99)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                | 38us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (46)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                             | (54)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 1,121us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                           | 1,199us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Corporate and other bonds [Member]                                                                                                                                                                             |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 162us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                    | 204us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                             | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 12us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                 | 5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Sales                                                                                                                                                                                                          | (12)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                   | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                             | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 37us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                       | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                       | (16)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 186us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                    | 189us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                                | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| States, municipalities and political subdivisions [Member]                                                                                                                                                     |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 94us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                            | 71us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                    | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Purchases                                                                                                                                                                                                      | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Sales                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Settlements                                                                                                                                                                                                    | (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                               | 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Transfers out of Level 3                                                                                                                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                              | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 86us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                            | 86us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                       | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Residential mortgage-backed [Member]                                                                                                                                                                           |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 189us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                        | 331us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                    | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                 | 15us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 72us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                     | 25us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                 | (21)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                            | 21us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (20)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                        | (13)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 232us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                        | 359us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                                    | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Commercial Mortgage Backed Securities [Member]                                                                                                                                                                 |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 83us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                          | 151us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                     | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                           | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                   | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (26)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                         | (24)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 64us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                          | 126us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                     | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Other asset-backed [Member]                                                                                                                                                                                    |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 655us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                        | 446us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                    | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Purchases                                                                                                                                                                                                      | 35us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                     | 148us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Sales                                                                                                                                                                                                          | (144)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                      | (83)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Settlements                                                                                                                                                                                                    | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                  | (72)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Transfers out of Level 3                                                                                                                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                           | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 553us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                        | 439us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                    | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Total asset-backed [Member]                                                                                                                                                                                    |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 927us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                      | 928us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                  | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 10us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                              | 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 113us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                  | 173us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | (144)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                    | (83)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                               | (94)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                          | 21us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (46)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                      | (38)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 849us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                      | 924us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                                  | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Equity securities [Member]                                                                                                                                                                                     |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 16us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                            | 11us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                       | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                  | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Purchases                                                                                                                                                                                                      | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Sales                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                             | (8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Settlements                                                                                                                                                                                                    | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                               | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                              | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 13us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                            | 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                                       | 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Life settlement contracts, included in Other assets [Member]                                                                                                                                                   |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 82us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                    | 88us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 13us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                              | 10us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Purchases                                                                                                                                                                                                      | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                 | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Sales                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Settlements                                                                                                                                                                                                    | (16)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                            | (11)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Transfers into Level 3                                                                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Transfers out of Level 3                                                                                                                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                      | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 79us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                    | 87us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 1cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                               | 1cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Separate account business [Member]                                                                                                                                                                             |                                                                                                                                                                                                                                     |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                     |   |
| Balance, Beginning, Assets                                                                                                                                                                                     |                                                                                                                                                                                                                                     | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Purchases                                                                                                                                                                                                      |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Sales                                                                                                                                                                                                          |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Settlements                                                                                                                                                                                                    |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Transfers into Level 3                                                                                                                                                                                         |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Transfers out of Level 3                                                                                                                                                                                       |                                                                                                                                                                                                                                     | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Balance, Ending, Assets                                                                                                                                                                                        |                                                                                                                                                                                                                                     | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         |                                                                                                                                                                                                                                     | $ 0cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |

| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unrealized gains (losses) Recognized in Net Income on Level 3 Assets and Liabilities Held
+ References
No definition available.
+ Details**Name:**cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld**Namespace Prefix:**cna_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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