| **Consolidated Statements of Comprehensive Income (Parenthetical) (USD $)In Millions, unless otherwise specified** | 12 Months Ended                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------|
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| **Statement of Comprehensive Income [Abstract]**                                                                   |                                                                                                                                   |   |   |
| Unrealized gains on investments available-for-sale, net of tax                                                     | $ 134us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax                                       | $ 161us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax | $ 135us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodTax |
| Amortization of pension actuarial loss and prior service cost, net of tax                                          | (6)cinf_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetActuarialLossAndPriorServiceCostArisingDuringPeriodTax | 29cinf_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetActuarialLossAndPriorServiceCostArisingDuringPeriodTax | (4)cinf_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetActuarialLossAndPriorServiceCostArisingDuringPeriodTax |
| Change in life deferred acquisition costs, life policy reserves and other, net of tax                              | $ 2cinf_OtherComprehensiveIncomeChangeInLifeDeferredAcquisitionCostsLifePolicyReservesAndOtherTax                                      | $ 12cinf_OtherComprehensiveIncomeChangeInLifeDeferredAcquisitionCostsLifePolicyReservesAndOtherTax | $ (7)cinf_OtherComprehensiveIncomeChangeInLifeDeferredAcquisitionCostsLifePolicyReservesAndOtherTax |

| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax
+ References
No definition available.
+ Details**Name:**cinf_OtherComprehensiveIncomeChangeInLifeDeferredAcquisitionCostsLifePolicyReservesAndOtherTax**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax
+ References
No definition available.
+ Details**Name:**cinf_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetActuarialLossAndPriorServiceCostArisingDuringPeriodTax**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e,f) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580
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| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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