| **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** | 12 Months Ended                                                                                                                                                                                                                            |
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| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | $ 1,303us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                            | $ 1,393us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 27us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                                                                           | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 46us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss                                                                                                        |   |
| Purchases                                                                                                                                                                                                      | 673us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases                                                                                                                                            | 804us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases |
| Sales                                                                                                                                                                                                          | (386)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales                                                                                                                                              | (414)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales |
| Settlements                                                                                                                                                                                                    | (273)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements                                                                                                                                        | (275)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements |
| Transfers into Level 3                                                                                                                                                                                         | 107us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3                                                                                                                                  | 94us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 |
| Transfers out of Level 3                                                                                                                                                                                       | (216)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3                                                                                                                               | (294)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 |
| Balance, Ending, Assets                                                                                                                                                                                        | 1,281us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                              | 1,303us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Unrealized gains (losses) on Level 3 assets and liabilities held at balance sheet date recognized in net income (loss)                                                                                         | 7cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld                                                                                                                                                            | (36)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld |
| Fixed maturity securities [Member]                                                                                                                                                                             |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 1,203us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                             | 1,251us-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)                                                                                  | (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                         | 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 52us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                       | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 657us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                           | 801us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Sales                                                                                                                                                                                                          | (378)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                             | (405)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (234)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                       | (251)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 107us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                 | 94us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (215)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                              | (293)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 1,183us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                             | 1,203us-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)                                                                                         | (1)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_DebtSecuritiesMember                                                                                         | (7)cna_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                                                                                                                                                                                     | 204us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                      | 219us-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)                                                                                  | 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                  | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                             |   |
| Purchases                                                                                                                                                                                                      | 33us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                   | 142us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Sales                                                                                                                                                                                                          | (23)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                     | (116)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (16)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                               | (44)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 18us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                         | 51us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (55)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                      | (51)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 162us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CorporateDebtSecuritiesMember                                                                      | 204us-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)                                                                                         |                                                                                                                                                                                                                                       | (2)cna_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                                                                                                                                                                                     | 71us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                              | 96us-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                                         | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                      | 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Purchases                                                                                                                                                                                                      | 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                          | 122us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Sales                                                                                                                                                                                                          | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                            | (79)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Settlements                                                                                                                                                                                                    |                                                                                                                                                                                                                                       | (61)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Transfers into Level 3                                                                                                                                                                                         | 14us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                | 18us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Transfers out of Level 3                                                                                                                                                                                       |                                                                                                                                                                                                                                       | (27)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 94us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                              | 71us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ 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                                                                                                                                                                                     | 331us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                          | 413us-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)                                                                                  | (21)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                   | 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 61us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                  | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 94us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                       | 116us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | (174)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                        | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (72)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                   | (75)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 32us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                             | 4us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (62)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                          | (107)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 189us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ResidentialMortgageBackedSecuritiesMember                                                          | 331us-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)                                                                                         |                                                                                                                                                                                                                                       | (3)cna_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                                                                                                                                                                                     | 151us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                           | 129us-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)                                                                                  | 7us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                       |   |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                  | 11us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 28us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                        | 107us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | (60)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                          | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (29)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                    | (11)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 43us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                              | 21us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (51)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                           | (103)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 83us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                            | 151us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ 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                                                                                                                                                                                     | 446us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                          | 368us-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)                                                                                  | 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                      | 5us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Purchases                                                                                                                                                                                                      | 488us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                      | 314us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Sales                                                                                                                                                                                                          | (111)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                        | (197)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Settlements                                                                                                                                                                                                    | (117)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                  | (35)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (47)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                          | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 655us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                          | 446us-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)                                                                                         | (1)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember                                                    | (2)cna_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                                                                                                                                                                                     | 928us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                        | 910us-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)                                                                                  | (12)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                 | 9us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | 49us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                | (7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Purchases                                                                                                                                                                                                      | 610us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                    | 537us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Sales                                                                                                                                                                                                          | (345)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                      | (210)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Settlements                                                                                                                                                                                                    | (218)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                | (121)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Transfers into Level 3                                                                                                                                                                                         | 75us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                           | 25us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Transfers out of Level 3                                                                                                                                                                                       | (160)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                       | (215)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 927us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                        | 928us-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)                                                                                         | (1)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember                                                                                  | (5)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_AssetBackedSecuritiesMember |
| Redeemable Preferred Stock [Member]                                                                                                                                                                            |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     |                                                                                                                                                                                                                                       | 26us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_RedeemablePreferredStockMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  |                                                                                                                                                                                                                                       | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_RedeemablePreferredStockMember |
| Settlements                                                                                                                                                                                                    |                                                                                                                                                                                                                                       | (25)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_RedeemablePreferredStockMember |
| Equity securities [Member]                                                                                                                                                                                     |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 11us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                              | 34us-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)                                                                                  | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                         | (27)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Net change in unrealized appreciation (depreciation) included in other comprehensive income (loss)                                                                                                             | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                    | 3us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Purchases                                                                                                                                                                                                      | 16us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                          | 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Sales                                                                                                                                                                                                          | (8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                             |   |
| Transfers out of Level 3                                                                                                                                                                                       |                                                                                                                                                                                                                                       | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 16us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                              | 11us-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)                                                                                         |                                                                                                                                                                                                                                       | (27)cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Other invested assets, including derivatives, net                                                                                                                                                              |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Sales                                                                                                                                                                                                          |                                                                                                                                                                                                                                       | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= cna_DerivativeAndOtherFinancialInstrumentsNetMember |
| Settlements                                                                                                                                                                                                    |                                                                                                                                                                                                                                       | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_DerivativeAndOtherFinancialInstrumentsNetMember |
| Short-term Investments [Member]                                                                                                                                                                                |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     |                                                                                                                                                                                                                                       | 6us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_ShortTermInvestmentsMember |
| Sales                                                                                                                                                                                                          |                                                                                                                                                                                                                                       | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_ShortTermInvestmentsMember |
| Life settlement contracts, included in Other assets [Member]                                                                                                                                                   |                                                                                                                                                                                                                                       |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                              |                                                                                                                                                                                                                                       |   |
| Balance, Beginning, Assets                                                                                                                                                                                     | 88us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                      | 100us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)                                                                                  | 33us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                | 13us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Settlements                                                                                                                                                                                                    | (39)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                              | (25)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember |
| Balance, Ending, Assets                                                                                                                                                                                        | 82us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                      | 88us-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)                                                                                         | 8cna_UnrealizedGainsLossesRecognizedInNetIncomeOnLevel3AssetsAndLiabilitiesHeld/ us-gaap_FairValueByAssetClassAxis= cna_LifeSettlementContractsIncludedInOtherAssetsMember                                                                 | (2)cna_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                                                                            | 2us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Purchases                                                                                                                                                                                                      |                                                                                                                                                                                                                                       | 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Sales                                                                                                                                                                                                          |                                                                                                                                                                                                                                       | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember |
| Transfers out of Level 3                                                                                                                                                                                       | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3/ us-gaap_FairValueByAssetClassAxis= cna_SeparateAccountBusinessMember                                                           |   |
| Balance, Ending, Assets                                                                                                                                                                                        |                                                                                                                                                                                                                                       | $ 1us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ 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                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| 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
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| 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
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| 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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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