| **Discontinued Operations Income Statement Information (Details) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                     |
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| **Revenues**                                                                                                      |                                                                               |   |
| Net Investment Income                                                                                             |                                                                               | $ 41cna_DisposalGroupIncludingDiscontinuedOperationNetInvestmentIncome |
| Net Realized Investment Gains (Losses)                                                                            |                                                                               | 1cna_DisposalGroupIncludingDiscontinuedOperationNetRealizedInvestmentGains |
| Total Revenues                                                                                                    |                                                                               | 42us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue |
| **Claims, Benefits and Expenses**                                                                                 |                                                                               |   |
| Insurance Claims and Policyholders' Benefits                                                                      |                                                                               | 31cna_DisposalGroupIncludingDiscontinuedOperationInsuranceClaimsandPolicyholdersBenefits |
| Other Operating Expenses                                                                                          |                                                                               | 1cna_DisposalGroupIncludingDiscontinuedOperationOtherOperatingExpenses |
| Total Claims, Benefits and Expenses                                                                               |                                                                               | 32us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingExpense |
| Income before Income Tax                                                                                          |                                                                               | 10us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodBeforeIncomeTax |
| Income tax (expense) benefit                                                                                      |                                                                               | (3)us-gaap_DiscontinuedOperationTaxEffectOfIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriod |
| Income from operations of discontinued operations, net of income tax                                              |                                                                               | 7us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationDuringPhaseOutPeriodNetOfTax |
| Impairment loss on sale, net of income tax benefit                                                                |                                                                               | (214)us-gaap_DiscontinuedOperationProvisionForLossGainOnDisposalNetOfTax |
| Income (Loss) from Discontinued Operations                                                                        | 0us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity | (207)us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTaxAttributableToReportingEntity |
| Income tax benefit (expense) on impairment                                                                        |                                                                               | $ 41us-gaap_DiscontinuedOperationTaxExpenseBenefitFromProvisionForGainLossOnDisposal |

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Disposal Group Including Discontinued Operation Insurance Claims and Policyholders Benefits
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Disposal Group Including Discontinued Operation Net Investment Income
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Disposal Group Including Discontinued Operation Net Realized Investment Gains
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Disposal Group Including Discontinued Operation Other Operating Expenses
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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.
+ References
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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.
+ References
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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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