| **Employee Retirement Benefits (Components Of Net Periodic Benefit Cost As Well As Other Changes In Plan Assets And Benefit Obligations Recognized In Other Comprehensive Income For Qualified Plan And SERP) (Details) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                         |
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| **Compensation and Retirement Disclosure [Abstract]**                                                                                                                                                                                                                    |                                                                                                                                    |   |   |
| Service cost                                                                                                                                                                                                                                                             | $ 10us-gaap_DefinedBenefitPlanServiceCost                                                                                               | $ 13us-gaap_DefinedBenefitPlanServiceCost | $ 12us-gaap_DefinedBenefitPlanServiceCost |
| Interest cost                                                                                                                                                                                                                                                            | 15us-gaap_DefinedBenefitPlanInterestCost                                                                                                | 13us-gaap_DefinedBenefitPlanInterestCost | 14us-gaap_DefinedBenefitPlanInterestCost |
| Expected return on plan assets                                                                                                                                                                                                                                           | (17)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets                                                                                | (17)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets | (16)us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssets |
| Amortization of actuarial loss and prior service cost                                                                                                                                                                                                                    | 2cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit                                                  | 9cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit | 8cinf_DefinedBenefitPlanAmortizationOfNetActuarialGainLossAndPriorServiceCostOrBenefit |
| Other                                                                                                                                                                                                                                                                    | 3cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts                                                                                    | 2cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts | 0cinf_DefinedBenefitPlanOtherNetPeriodicBenefitCosts |
| Net periodic benefit cost                                                                                                                                                                                                                                                | 13us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost                                                                                      | 20us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost | 18us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost |
| Current year actuarial loss (gain)                                                                                                                                                                                                                                       | 21us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax       | (72)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax | 20us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetUnamortizedGainLossArisingDuringPeriodBeforeTax |
| Amortization of actuarial loss                                                                                                                                                                                                                                           | 5us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax | 10us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax | 6us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossBeforeTax |
| Other                                                                                                                                                                                                                                                                    | 2us-gaap_DefinedBenefitPlanOtherCosts                                                                                                   | 0us-gaap_DefinedBenefitPlanOtherCosts | 0us-gaap_DefinedBenefitPlanOtherCosts |
| Amortization of prior service cost                                                                                                                                                                                                                                       | 0us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax                                | (1)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax | (1)us-gaap_DefinedBenefitPlanAmountsRecognizedInOtherComprehensiveIncomeNetPriorServiceCostCreditBeforeTax |
| Total recognized in other comprehensive loss (income)                                                                                                                                                                                                                    | $ 18us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax                                    | $ (83)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax | $ 13us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentBeforeTax |

| X                                                                                                                                                                                                                                                                                                                                                                                        |
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| - Definition
The amount of actuarial gains or losses and prior service cost or credit recognized in net periodic benefit cost.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan, Other Net Periodic Benefit Costs
+ References
No definition available.
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| - Definition
Amount before tax, after reclassification adjustments, of prior service cost (credit) that has not been recognized in the income statement.
+ References
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| - Definition
An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.
+ References
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| - Definition
The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.
+ References
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| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other defined benefit plan costs.
+ References
No definition available.
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| - Definition
The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax, after reclassification adjustments, of (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.
+ References
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