| **Investments - Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $)In Millions, unless otherwise specified** | Mar. 31, 2015                                                                                                                                                                                                                           | Dec. 31, 2014                                                                                                                                                                                                                           |
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| Debt Securities                                                                                                                                                                |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | $ 8,961us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              | $ 8,871us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              |
| Gross Unrealized Gains                                                                                                                                                         | 648cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              | 607cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                              |
| Gross Unrealized Losses                                                                                                                                                        | 13cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                               | 18cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                               |
| Fair Value                                                                                                                                                                     | 9,596us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                             | 9,460us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember                                                                                                                             |
| Debt Securities | Corporate                                                                                                                                                    |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 5,152us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                        | 5,117us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                        |
| Gross Unrealized Gains                                                                                                                                                         | 449cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                      | 420cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                      |
| Gross Unrealized Losses                                                                                                                                                        | 8cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                        | 11cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                       |
| Fair Value                                                                                                                                                                     | 5,593us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                                     | 5,526us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateBondSecuritiesMember                                     |
| Debt Securities | States, municipalities and political subdivisions                                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 3,289us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember               | 3,267us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember               |
| Gross Unrealized Gains                                                                                                                                                         | 183cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember             | 178cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember             |
| Gross Unrealized Losses                                                                                                                                                        | 2cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember               | 2cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember               |
| Fair Value                                                                                                                                                                     | 3,470us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                            | 3,443us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USStatesAndPoliticalSubdivisionsMember                            |
| Debt Securities | Commercial mortgage-backed                                                                                                                                   |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 269us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               | 250us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember               |
| Gross Unrealized Gains                                                                                                                                                         | 16cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember            | 9cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember             |
| Gross Unrealized Losses                                                                                                                                                        | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember             | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember             |
| Fair Value                                                                                                                                                                     | 285us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                            | 259us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommercialMortgageBackedSecuritiesMember                            |
| Debt Securities | Government-sponsored enterprises                                                                                                                             |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 227us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   | 213us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember   |
| Gross Unrealized Gains                                                                                                                                                         | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember |
| Gross Unrealized Losses                                                                                                                                                        | 3cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember | 5cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember |
| Fair Value                                                                                                                                                                     | 224us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                | 208us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember                |
| Debt Securities | Foreign government                                                                                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 10us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                   | 10us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                   |
| Gross Unrealized Gains                                                                                                                                                         | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                |
| Gross Unrealized Losses                                                                                                                                                        | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                |
| Fair Value                                                                                                                                                                     | 10us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                | 10us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ForeignGovernmentDebtSecuritiesMember                                |
| Debt Securities | Convertibles and bonds with warrants attached                                                                                                                |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 7us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember           | 7us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember           |
| Gross Unrealized Gains                                                                                                                                                         | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember       | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember       |
| Gross Unrealized Losses                                                                                                                                                        | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember       | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember       |
| Fair Value                                                                                                                                                                     | 7us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember                        | 7us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_ConvertiblesAndBondsWithWarrantsAttachedMember                        |
| Debt Securities | United States government                                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 7us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                 | 7us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                 |
| Gross Unrealized Gains                                                                                                                                                         | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember             | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember             |
| Gross Unrealized Losses                                                                                                                                                        | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember             | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember             |
| Fair Value                                                                                                                                                                     | 7us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                              | 7us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_DebtSecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_USGovernmentAgenciesDebtSecuritiesMember                              |
| Equity Securities                                                                                                                                                              |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 2,748us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                              | 2,728us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                              |
| Gross Unrealized Gains                                                                                                                                                         | 2,052cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                          | 2,134cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                          |
| Gross Unrealized Losses                                                                                                                                                        | 11cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                             | 4cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                              |
| Fair Value                                                                                                                                                                     | 4,789us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                                           | 4,858us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember                                                                                                                           |
| Equity Securities | Common equities                                                                                                                                            |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 2,595us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  | 2,583us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  |
| Gross Unrealized Gains                                                                                                                                                         | 2,014cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                              | 2,099cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                              |
| Gross Unrealized Losses                                                                                                                                                        | 11cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                 | 3cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                  |
| Fair Value                                                                                                                                                                     | 4,598us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                               | 4,679us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CommonStockMember                                               |
| Equity Securities | Nonredeemable preferred equities                                                                                                                           |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 153us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                 | 145us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                 |
| Gross Unrealized Gains                                                                                                                                                         | 38cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                              | 35cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                              |
| Gross Unrealized Losses                                                                                                                                                        | 0cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               | 1cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                               |
| Fair Value                                                                                                                                                                     | 191us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                              | 179us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= us-gaap_EquitySecuritiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_PreferredStockMember                                              |
| Fixed Maturities And Equity Securities                                                                                                                                         |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| **Schedule of Available-for-sale Securities [Line Items]**                                                                                                                     |                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                    |
| Cost or Amortized Cost                                                                                                                                                         | 11,709us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                              | 11,599us-gaap_AvailableForSaleSecuritiesAmortizedCost/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                              |
| Gross Unrealized Gains                                                                                                                                                         | 2,700cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                           | 2,741cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                           |
| Gross Unrealized Losses                                                                                                                                                        | 24cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                              | 22cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                              |
| Fair Value                                                                                                                                                                     | $ 14,385us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                                         | $ 14,318us-gaap_AvailableForSaleSecurities/ invest_InvestmentSecuritiesClassAxis= cinf_FixedMaturitiesAndEquitySecuritiesMember                                                                                                         |

| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesGrossUnrealizedLoss2**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
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| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
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| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
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| - Details**Name:**us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
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| - Details**Name:**invest_InvestmentSecuritiesClassAxis=cinf_FixedMaturitiesAndEquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |