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Investments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Gain (Loss) on Investments      
Largest equity holdings to total equity portfolio 3.30%cinf_Largestequityholdingstototalequityportfolio    
Largest equity holdings to total portfolio 1.10%cinf_Largestequityholdingstototalportfolio    
Investments, deposit with various states $ 75us-gaap_DepositAssets $ 80us-gaap_DepositAssets  
Investments in fair value, deposit with various states 15,067us-gaap_InvestmentsFairValueDisclosure 14,192us-gaap_InvestmentsFairValueDisclosure  
Impaired securities 6cinf_NumberOfSecuritiesOtherThanTemporaryImpairmentLosses 7cinf_NumberOfSecuritiesOtherThanTemporaryImpairmentLosses 13cinf_NumberOfSecuritiesOtherThanTemporaryImpairmentLosses
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss 10cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLossAccumulatedinInvestments 5cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLossAccumulatedinInvestments 1cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLossAccumulatedinInvestments
Fixed Maturities      
Gain (Loss) on Investments      
Number of securities total unrealized loss in an unrealized loss position for 12 months or more 144cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_DebtSecuritiesMember
40cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_DebtSecuritiesMember
4cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_DebtSecuritiesMember
Equity Securities      
Gain (Loss) on Investments      
Number of securities total unrealized loss in an unrealized loss position for 12 months or more 3cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_EquitySecuritiesMember
0cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_EquitySecuritiesMember
0cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_EquitySecuritiesMember
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss 1cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLossAccumulatedinInvestments
/ invest_InvestmentSecuritiesClassAxis
= us-gaap_EquitySecuritiesMember
   
Exxon Mobil Corporation, The Procter & Gamble Company, Honeywell International Inc, BlackRock Inc., Genuine Parts Company, RPM International, and Johnson and Johnson [Member]      
Gain (Loss) on Investments      
Available-for-sale securities, accumulated net gain position 619us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ dei_LegalEntityAxis
= cinf_ProcterGambleCompanyExxonMobilCorporationAndChevronCorporationMember
   
Seven Primary Contributors      
Gain (Loss) on Investments      
Number of securities 7cinf_NumberOfSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_InvestmentsMember
   
On Deposit With Various States      
Gain (Loss) on Investments      
Investments in fair value, deposit with various states $ 78us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_InvestmentSecondaryCategorizationAxis
= cinf_Group3Member
$ 84us-gaap_InvestmentsFairValueDisclosure
/ us-gaap_InvestmentSecondaryCategorizationAxis
= cinf_Group3Member