| **Income Taxes (Narrative) (Details) (USD $)**                     | 12 Months Ended                                                                                                              |
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| **Income Tax Disclosure [Abstract]**                               |                                                                                                                         |   |   |
| Federal income taxes received from (paid to) Loews                 | $ (287,000,000)cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount                                                           | $ (89,000,000)cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount | $ 75,000,000cna_FederalIncomeTaxesReceivedFromPaidToLoewsAmount |
| Unrecognized tax benefits                                          | 0us-gaap_UnrecognizedTaxBenefits                                                                                             | 0us-gaap_UnrecognizedTaxBenefits |   |
| Interest income (expense)                                          | 0us-gaap_InterestIncomeExpenseNet                                                                                            | 0us-gaap_InterestIncomeExpenseNet | 2,000,000us-gaap_InterestIncomeExpenseNet |
| Unrecognized Tax Benefits, Income Tax Penalties Expense            | 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense                                                                    | 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense | 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesExpense |
| Accrued interest and penalties                                     | 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued                                                         | 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued |   |
| Deferred tax liabilities not recognized on undistributed earnings  | 0us-gaap_DeferredTaxLiabilityNotRecognizedAmountOfUnrecognizedDeferredTaxLiabilityUndistributedEarningsOfForeignSubsidiaries |   |   |
| Foreign tax expense (benefit) on income from continuing operations | 24,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations                                                         | 24,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations | 34,000,000us-gaap_ForeignIncomeTaxExpenseBenefitContinuingOperations |
| Income (loss) from continuing foreign operations                   | 66,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign                                                 | 101,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign | 88,000,000us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesForeign |
| Tax Credit Carryforward, Amount                                    | 0us-gaap_TaxCreditCarryforwardAmount                                                                                         |   |   |
| Valuation allowance                                                | $ 0us-gaap_DeferredTaxAssetsValuationAllowance                                                                               | $ 0us-gaap_DeferredTaxAssetsValuationAllowance |   |

| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Federal income taxes received from (paid to) Loews amount
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax liability not recognized because of the exceptions to comprehensive recognition of deferred taxes related to undistributed earnings of foreign subsidiaries.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6424122&loc=d3e41874-109331
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of current and deferred foreign income tax expense (benefit) attributable to income (loss) from continuing operations.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)(1)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 1 -Article 4
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The portion of earnings or loss from continuing operations before income taxes that is attributable to foreign operations, which is defined as Income or Loss generated from operations located outside the entity's country of domicile.
+ References
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| X                                                                                                                                                               |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net amount of operating interest income (expense).
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense for penalties related to a tax position claimed or expected to be claimed in the tax return.
+ References
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