| **Consolidated Statements of Comprehensive Income (Loss) (USD $)In Millions, unless otherwise specified**                                                           | 12 Months Ended                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Other Comprehensive Income (Loss), Net of Tax, Changes In:**                                                                                                      |                                                                                                                                                        |  |   |   |
| Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Portion Attributable to Parent, Available-for-sale Securities | $ 15us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAvailableforsaleSecurities |  | $ 6us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAvailableforsaleSecurities | $ 84us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParentAvailableforsaleSecurities |
| Net unrealized gains (losses) on other investments                                                                                                                  | 267us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax                                                                         |  | (679)us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax | 339us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax |
| Net unrealized gains (losses) on investments                                                                                                                        | 282cna_NetUnrealizedGainsLossesOnInvestments                                                                                                                |  | (673)cna_NetUnrealizedGainsLossesOnInvestments | 423cna_NetUnrealizedGainsLossesOnInvestments |
| Net unrealized losses on discontinued operations                                                                                                                    | (22)cna_Netunrealizedgainslossesondiscontinuedoperationsandotheraftertax                                                                                    |  |   |   |
| Foreign currency translation adjustment                                                                                                                             | (95)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax                                                          |  | (11)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax | 40us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax |
| Pension and postretirement benefits                                                                                                                                 | (207)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax                                                        |  | 295us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax | (112)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax |
| Other comprehensive income (loss), net of tax                                                                                                                       | (42)us-gaap_OtherComprehensiveIncomeLossNetOfTax                                                                                                            |  | (389)us-gaap_OtherComprehensiveIncomeLossNetOfTax | 351us-gaap_OtherComprehensiveIncomeLossNetOfTax |
| Net income (loss)                                                                                                                                                   | 691us-gaap_NetIncomeLoss                                                                                                                                    | [1] | 937us-gaap_NetIncomeLoss | 628us-gaap_NetIncomeLoss |
| Total comprehensive income (loss)                                                                                                                                   | $ 649us-gaap_ComprehensiveIncomeNetOfTax                                                                                                                    |  | $ 548us-gaap_ComprehensiveIncomeNetOfTax | $ 979us-gaap_ComprehensiveIncomeNetOfTax |
|                                                                                                                                                                     |
| [1]Net income in the first quarter of 2014 includes the impairment loss on the sale of CAC.                                                                         |

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Net unrealized gains (losses) on discontinued operations and other and other, after tax.
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Unrealized change in value of total investments, after tax.
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Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.
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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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Amount after tax and reclassification adjustments, of appreciation (loss) in value of unsold available-for-sale securities. Excludes amounts related to other than temporary impairment (OTTI) loss.
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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.
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Amount after tax and reclassification adjustments of other comprehensive income (loss).
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Amount after tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to parent entity.
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