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Fair Value Measurements - Summary of Book Value and Principal Amounts of Long-Term Debt (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Debt Instrument [Line Items]    
Book value $ 791us-gaap_LongTermDebt $ 791us-gaap_LongTermDebt
Principal amount of debt 793us-gaap_DebtInstrumentFaceAmount 793us-gaap_DebtInstrumentFaceAmount
Senior Debentures 6.900% Due 2028    
Debt Instrument [Line Items]    
Interest rate 6.90%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember		 6.90%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember
Debt issuance year 1998 1998
Book value 28us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember		 28us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember
Principal amount of debt 28us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember		 28us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNinePercentDueTwentyTwentyEightMember
Senior Debentures 6.920% Due 2028    
Debt Instrument [Line Items]    
Interest rate 6.92%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember		 6.92%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember
Debt issuance year 2005 2005
Book value 391us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember		 391us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember
Principal amount of debt 391us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember		 391us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorDebenturesSixPointNineTwoPercentDueTwentyTwentyEightMember
Senior Notes 6.152% Due 2034    
Debt Instrument [Line Items]    
Interest rate 6.125%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember		 6.125%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember
Debt issuance year 2004 2004
Book value 372us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember		 372us-gaap_LongTermDebt
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember
Principal amount of debt $ 374us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember		 $ 374us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= cinf_SeniorNotesSixPointOneTwoFivePercentDueTwentyThirtyFourMember