| **Discontinued Operations Income Statement Information (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                       |
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| **Revenues**                                                                                                      |                                                                                                  |   |   |
| Net Earned Premium                                                                                                |                                                                                                  |   | $ 1cna_DisposalGroupIncludingDiscontinuedOperationsNetEarnedPremium |
| Net Investment Income                                                                                             | 94cna_DisposalGroupIncludingDiscontinuedOperationNetInvestmentIncome                                  | 168cna_DisposalGroupIncludingDiscontinuedOperationNetInvestmentIncome | 172cna_DisposalGroupIncludingDiscontinuedOperationNetInvestmentIncome |
| Net Realized Investment Gains (Losses)                                                                            | 3cna_DisposalGroupIncludingDiscontinuedOperationNetRealizedInvestmentGains                            | 11cna_DisposalGroupIncludingDiscontinuedOperationNetRealizedInvestmentGains | 9cna_DisposalGroupIncludingDiscontinuedOperationNetRealizedInvestmentGains |
| Other Revenues                                                                                                    |                                                                                                  | 2cna_DisposalGroupIncludingDiscontinuedOperationOtherRevenues | 1cna_DisposalGroupIncludingDiscontinuedOperationOtherRevenues |
| Total Revenues                                                                                                    | 97us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue                                          | 181us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue | 183us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue |
| **Claims, Benefits and Expenses**                                                                                 |                                                                                                  |   |   |
| Insurance Claims and Policyholders' Benefits                                                                      | 75cna_DisposalGroupIncludingDiscontinuedOperationInsuranceClaimsandPolicyholdersBenefits              | 141cna_DisposalGroupIncludingDiscontinuedOperationInsuranceClaimsandPolicyholdersBenefits | 167cna_DisposalGroupIncludingDiscontinuedOperationInsuranceClaimsandPolicyholdersBenefits |
| Other Operating Expenses                                                                                          | 2cna_DisposalGroupIncludingDiscontinuedOperationOtherOperatingExpenses                                | 3cna_DisposalGroupIncludingDiscontinuedOperationOtherOperatingExpenses | 3cna_DisposalGroupIncludingDiscontinuedOperationOtherOperatingExpenses |
| Total Claims, Benefits and Expenses                                                                               | 77us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense                                 | 144us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense | 170us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense |
| Income before Income Tax                                                                                          | 20us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax | 37us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax | 13us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax |
| Income tax (expense) benefit                                                                                      | (6)us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod    | (15)us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod | (5)us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod |
| Income from operations of discontinued operations, net of income tax                                              | 14us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax        | 22us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax | 8us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax |
| Loss on sale, net of income tax (expense) benefit of $40, - and -                                                 | (211)us-gaap_DiscontinuedOperationProvisionForLossGainOnDisposalNetOfTax                              |   |   |
| Income (Loss) from Discontinued Operations                                                                        | (197)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity                | 22us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | 8us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity |
| Income tax benefit (expense) on impairment                                                                        | $ 40us-gaap_DiscontinuedOperationTaxExpenseBenefitFromProvisionForGainLossOnDisposal                  |   |   |

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Disposal Group Including Discontinued Operation Net Investment Income
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Disposal Group Including Discontinued Operations Net Earned Premium
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| - Definition
Pretax income (loss) from operations of a business component (exclusive of any gain (loss) on disposal, or provision therefore) during the reporting period, until its disposal.
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| - Definition
After tax income (loss) from operations of a business component (exclusive of any gain (loss) on disposal, or provision therefore) during the reporting period, until its disposal.
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| - Definition
Amount after tax of loss recognized for the write-down to fair value, less cost to sell, of a disposal group. Includes after tax gain for a subsequent increase in fair value (less cost to sell) up to the cumulative loss previously recognized through write-downs.
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| - Definition
Income tax expense or benefit attributable to Income or Loss from discontinued operations during the phase-out period.
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| - Definition
Tax benefit from provision recognizing a loss for initial, or subsequent, write-down to fair value (less cost to sell) of a of a disposal group that is classified as a component of the entity, which remains unsold as of the reporting date. May include a tax on the (gain) for the recognition of a subsequent increase in fair value (less cost to sell), but not in excess of the cumulative loss previously recognized through write-downs.
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| - Definition
Amount of operating expenses attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.
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| - Definition
Amount of sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.
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| - Definition
Amount of income (loss) from a disposal group, net of income tax, reported as a separate component of income before extraordinary items after deduction or consideration of the amount allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.
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