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Employee Retirement Benefits - Defined Contribution Plan (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Defined Benefit Plan Disclosure [Line Items]    
Matching contributions to 401(k) and Top Hat plans $ 4us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount $ 4us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Qualified Pension Plans [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Expected contributions by employer in the remainder of year 0cinf_DefinedBenefitPlansEstimatedFutureEmployerContributionsinCurrentFiscalYear.
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember		  
Pension contributions $ 5us-gaap_PensionContributions
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember