| **Fair Value Measurements (Fair Value Of Deferred Annuities, Structured Settlements And Other Items) (Details) (USD $)In Millions, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                                                                               | Dec. 31, 2013                                                                                                                                                                                                               |
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| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | $ 1,114us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure                                                                                                                                                   | $ 1,130us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure                                                                                                                                                   |
| Deferred Annuities                                                                                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 897us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember                                                                                         | 911us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember                                                                                         |
| Structured Settlements                                                                                                                                          |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 217us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember                                                                                      | 219us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember                                                                                      |
| Level 1                                                                                                                                                         |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                    | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member                                                                    |
| Level 1 | Deferred Annuities                                                                                                                                    |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member      | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member      |
| Level 1 | Structured Settlements                                                                                                                                |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member   | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel1Member   |
| Level 2                                                                                                                                                         |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 217us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                  | 219us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member                                                                  |
| Level 2 | Deferred Annuities                                                                                                                                    |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member      | 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member      |
| Level 2 | Structured Settlements                                                                                                                                |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 217us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member | 219us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel2Member |
| Level 3                                                                                                                                                         |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 897us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                  | 911us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                                  |
| Level 3 | Deferred Annuities                                                                                                                                    |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | 897us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member    | 911us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= cinf_DeferredAnnuitiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member    |
| Level 3 | Structured Settlements                                                                                                                                |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| **Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis**                                                                              |                                                                                                                                                                                                                        |                                                                                                                                                                                                                        |
| Fair value of deferred annuities and structured settlements                                                                                                     | $ 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member | $ 0us-gaap_LiabilitiesRelatedToInvestmentContractsFairValueDisclosure/ us-gaap_DerivativeByNatureAxis= us-gaap_StructuredFinanceMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |

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| - Definition
Fair value portion of liability related to investment contracts for example, but not limited to, guaranteed investment contract or annuities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 825 -Section 25 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6487514&loc=d3e32427-158581Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 14 -Article 7
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