| **Fair Value Measurements - Fair Value Hierarchy for Assets Measured at Fair Value on Recurring Basis (Detail) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                                                                                                               | Dec. 31, 2014                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | $ 9,596us-gaap_AvailableForSaleSecuritiesDebtSecurities                                                                                                                                                                                                                                                                     | $ 9,460us-gaap_AvailableForSaleSecuritiesDebtSecurities                                                                                                                                                                                                                                                                     |
| Equity securities, at fair value                                                                                                                                | 4,789us-gaap_AvailableForSaleSecuritiesEquitySecurities                                                                                                                                                                                                                                                                     | 4,858us-gaap_AvailableForSaleSecuritiesEquitySecurities                                                                                                                                                                                                                                                                     |
| Short-term investments, at fair value (amortized cost: 2015—$25; 2014—$0)                                                                         | 25us-gaap_ShortTermInvestments                                                                                                                                                                                                                                                                                              | 0us-gaap_ShortTermInvestments                                                                                                                                                                                                                                                                                               |
| Separate accounts                                                                                                                                               | 764us-gaap_SeparateAccountAssets                                                                                                                                                                                                                                                                                            | 752us-gaap_SeparateAccountAssets                                                                                                                                                                                                                                                                                            |
| Fair value of asset                                                                                                                                             | 15,154us-gaap_InvestmentsFairValueDisclosure                                                                                                                                                                                                                                                                                | 15,067us-gaap_InvestmentsFairValueDisclosure                                                                                                                                                                                                                                                                                |
| Short-term investments                                                                                                                                          |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Short-term investments, at fair value (amortized cost: 2015—$25; 2014—$0)                                                                         | 25us-gaap_ShortTermInvestments/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                                                  |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                                                  | 19us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                                                                                                     | 18us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                                                                                                     |
| Debt Securities                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 9,596us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                                                                                                   | 9,460us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                                                                                                   |
| Debt Securities | Corporate                                                                                                                                     |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 5,593us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                                                                                                           | 5,526us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                                                                                                           |
| Debt Securities | States, municipalities and political subdivisions                                                                                             |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 3,470us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                                  | 3,443us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                                                                                                  |
| Debt Securities | Commercial mortgage-backed                                                                                                                    |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 285us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                                  | 259us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                                                                                                  |
| Debt Securities | Government-sponsored enterprises                                                                                                              |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 224us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                                                      | 208us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                                                                                      |
| Debt Securities | Foreign government                                                                                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 10us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                                      | 10us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                                                                                      |
| Debt Securities | Convertibles and bonds with warrants attached                                                                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember                                                                                              | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember                                                                                              |
| Debt Securities | United States government                                                                                                                      |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                                                    | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                                                                                                    |
| Equity Securities | Common equities                                                                                                                             |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 4,598us-gaap_AvailableForSaleSecuritiesEquitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                                   | 4,679us-gaap_AvailableForSaleSecuritiesEquitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                                                                                                   |
| Equity Securities | Nonredeemable preferred equities                                                                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 191us-gaap_AvailableForSaleSecuritiesEquitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                                  | 179us-gaap_AvailableForSaleSecuritiesEquitySecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                                                                                                  |
| Taxable Fixed Maturities                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Separate accounts                                                                                                                                               | 725us-gaap_SeparateAccountAssets/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                                                                                                                   | 731us-gaap_SeparateAccountAssets/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                                                                                                                   |
| Quoted prices in active markets for identical assets (Level 1)                                                                                                  |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fair value of asset                                                                                                                                             | 4,624us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                                                                                            | 4,704us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                                                                                                                                            |
| Quoted prices in active markets for identical assets (Level 1) | Short-term investments                                                                         |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Short-term investments, at fair value (amortized cost: 2015—$25; 2014—$0)                                                                         | 0us-gaap_ShortTermInvestments/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                     |                                                                                                                                                                                                                                                                                                                        |
| Quoted prices in active markets for identical assets (Level 1) | Top Hat savings plan mutual funds and common equity (included in Other assets)                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                                                  | 19us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                | 18us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities                                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                  | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                  |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Corporate                                                                    |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                          | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                          |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | States, municipalities and political subdivisions                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Commercial mortgage-backed                                                   |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Government-sponsored enterprises                                             |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Foreign government                                                           |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                  | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                  |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | Convertibles and bonds with warrants attached                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         |
| Quoted prices in active markets for identical assets (Level 1) | Debt Securities | United States government                                                     |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               |
| Quoted prices in active markets for identical assets (Level 1) | Equity Securities | Common equities                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 4,598us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                              | 4,679us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                              |
| Quoted prices in active markets for identical assets (Level 1) | Equity Securities | Nonredeemable preferred equities                                           |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               |
| Quoted prices in active markets for identical assets (Level 1) | Taxable Fixed Maturities                                                                       |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Separate accounts                                                                                                                                               | 0us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                                | 0us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                                |
| Significant other observable inputs (Level 2)                                                                                                                   |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fair value of asset                                                                                                                                             | 10,509us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                           | 10,343us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                                                                                                                                           |
| Significant other observable inputs (Level 2) | Short-term investments                                                                                          |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Short-term investments, at fair value (amortized cost: 2015—$25; 2014—$0)                                                                         | 25us-gaap_ShortTermInvestments/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                    |                                                                                                                                                                                                                                                                                                                        |
| Significant other observable inputs (Level 2) | Top Hat savings plan mutual funds and common equity (included in Other assets)                                  |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                                                  | 0us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                 | 0us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                 |
| Significant other observable inputs (Level 2) | Debt Securities                                                                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 9,570us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              | 9,435us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              |
| Significant other observable inputs (Level 2) | Debt Securities | Corporate                                                                                     |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 5,575us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                      | 5,508us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                      |
| Significant other observable inputs (Level 2) | Debt Securities | States, municipalities and political subdivisions                                             |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 3,469us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember             | 3,443us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember             |
| Significant other observable inputs (Level 2) | Debt Securities | Commercial mortgage-backed                                                                    |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 285us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember             | 259us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember             |
| Significant other observable inputs (Level 2) | Debt Securities | Government-sponsored enterprises                                                              |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 224us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 208us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember |
| Significant other observable inputs (Level 2) | Debt Securities | Foreign government                                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 10us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                 | 10us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                 |
| Significant other observable inputs (Level 2) | Debt Securities | Convertibles and bonds with warrants attached                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         | 7us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         |
| Significant other observable inputs (Level 2) | Debt Securities | United States government                                                                      |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               |
| Significant other observable inputs (Level 2) | Equity Securities | Common equities                                                                             |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  |
| Significant other observable inputs (Level 2) | Equity Securities | Nonredeemable preferred equities                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 189us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                             | 177us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                             |
| Significant other observable inputs (Level 2) | Taxable Fixed Maturities                                                                                        |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Separate accounts                                                                                                                                               | 725us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                              | 731us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                              |
| Significant unobservable inputs (Level 3)                                                                                                                       |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fair value of asset                                                                                                                                             | 21us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                               | 20us-gaap_InvestmentsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                                                                                                                                               |
| Significant unobservable inputs (Level 3) | Short-term investments                                                                                              |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Short-term investments, at fair value (amortized cost: 2015—$25; 2014—$0)                                                                         | 0us-gaap_ShortTermInvestments/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ShortTermInvestmentsMember                                                                                                                     |                                                                                                                                                                                                                                                                                                                        |
| Significant unobservable inputs (Level 3) | Top Hat savings plan mutual funds and common equity (included in Other assets)                                      |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Top Hat savings plan mutual funds and common equity (included in Other assets)                                                                                  | 0us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                 | 0us-gaap_OtherInvestmentNotReadilyMarketableFairValue/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_OtherPensionPlansPostretirementOrSupplementalPlansDefinedBenefitMember                                                 |
| Significant unobservable inputs (Level 3) | Debt Securities                                                                                                     |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 19us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                 | 18us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                 |
| Significant unobservable inputs (Level 3) | Debt Securities | Corporate                                                                                         |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 18us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                         | 18us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                         |
| Significant unobservable inputs (Level 3) | Debt Securities | States, municipalities and political subdivisions                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 1us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                 |
| Significant unobservable inputs (Level 3) | Debt Securities | Commercial mortgage-backed                                                                        |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               |
| Significant unobservable inputs (Level 3) | Debt Securities | Government-sponsored enterprises                                                                  |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   |
| Significant unobservable inputs (Level 3) | Debt Securities | Foreign government                                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                  | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                  |
| Significant unobservable inputs (Level 3) | Debt Securities | Convertibles and bonds with warrants attached                                                     |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember         |
| Significant unobservable inputs (Level 3) | Debt Securities | United States government                                                                          |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Fixed maturities, at fair value                                                                                                                                 | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               | 0us-gaap_AvailableForSaleSecuritiesDebtSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember               |
| Significant unobservable inputs (Level 3) | Equity Securities | Common equities                                                                                 |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  | 0us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  |
| Significant unobservable inputs (Level 3) | Equity Securities | Nonredeemable preferred equities                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Equity securities, at fair value                                                                                                                                | 2us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               | 2us-gaap_AvailableForSaleSecuritiesEquitySecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               |
| Significant unobservable inputs (Level 3) | Taxable Fixed Maturities                                                                                            |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                |                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                        |
| Separate accounts                                                                                                                                               | $ 0us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                              | $ 0us-gaap_SeparateAccountAssets/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member/ invest_InvestmentSecuritiesClassAxis= cinf_TaxableFixedMaturitiesMember                                                                                                                              |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of debt securities categorized neither as held-to-maturity nor trading.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of the other investment not readily marketable determined by management based upon fair value methods, including pricing of similar securities and valuation techniques, that was used to record the investment for financial reporting purposes.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The fair value of the assets held by the Entity for the benefit of separate account policyholders.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.
+ References
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