| **Income Taxes (Components Of Deferred Tax Assets And Liabilities) (Details) (USD $)In Millions, unless otherwise specified** | Dec. 31, 2014                                                                            | Dec. 31, 2013                                                                            |
|-------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------|
| **Deferred tax assets:**                                                                                                      |                                                                                     |                                                                                     |
| Loss and loss expense reserves                                                                                                | $ 197us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves          | $ 206us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLossReserves          |
| Unearned premiums                                                                                                             | 145us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve | 137us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsUnearnedPremiumsReserve |
| Investments                                                                                                                   | 16us-gaap_DeferredTaxAssetsInvestments                                                   | 19us-gaap_DeferredTaxAssetsInvestments                                                   |
| Other                                                                                                                         | 41us-gaap_DeferredTaxAssetsOther                                                         | 46us-gaap_DeferredTaxAssetsOther                                                         |
| Total                                                                                                                         | 399us-gaap_DeferredTaxAssetsNet                                                          | 408us-gaap_DeferredTaxAssetsNet                                                          |
| **Deferred tax liabilities:**                                                                                                 |                                                                                     |                                                                                     |
| Deferred tax liabilities:                                                                                                     | 937cinf_DeferredTaxLiabilitiesUnrealizedGainsAndLossesOnInvestments                      | 807cinf_DeferredTaxLiabilitiesUnrealizedGainsAndLossesOnInvestments                      |
| Deferred acquisition costs                                                                                                    | 183us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost            | 178us-gaap_DeferredTaxLiabilitiesDeferredExpenseDeferredPolicyAcquisitionCost            |
| Life policy reserves                                                                                                          | 110cinf_DeferredTaxLiabilitiesPolicyLiabilitiesLife                                      | 86cinf_DeferredTaxLiabilitiesPolicyLiabilitiesLife                                       |
| Other                                                                                                                         | 9us-gaap_DeferredTaxLiabilitiesOther                                                     | 10us-gaap_DeferredTaxLiabilitiesOther                                                    |
| Total                                                                                                                         | 1,239us-gaap_DeferredTaxLiabilities                                                      | 1,081us-gaap_DeferredTaxLiabilities                                                      |
| Net deferred income tax liability                                                                                             | $ 840us-gaap_DeferredTaxAssetsLiabilitiesNet                                             | $ 673us-gaap_DeferredTaxAssetsLiabilitiesNet                                             |

| X                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred Tax Liabilities Policy Liabilities Life
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| - Definition
Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from investments (excludes investments in subsidiaries and equity method investments).
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| - Definition
Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.
+ References
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| - Definition
Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.
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| - Definition
Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.
+ References
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| - Definition
Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from loss reserves other than estimated credit losses.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from the unearned premiums reserve.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount as of the balance sheet date of the estimated future tax effects attributable to policy acquisition costs being expensed for tax purposes but capitalized in conformity with generally accepted accounting principles, which will reverse in future periods when amortization of such capitalized costs cannot be deducted for tax purposes.
+ References
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| - Definition
Amount of deferred tax liability attributable to taxable temporary differences not separately disclosed.
+ References
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