| **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                                       |                                                                                                                                               |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss, Accumulated in Investments
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
largest equity holdings to total equity portfolio
+ References
No definition available.
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| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
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+ References
No definition available.
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| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities, Other Than Temporary Impairment Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 8 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8,17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560
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| X                                                                                                                                                                                                                                                                      |
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| X                                                                                                                                                                                     |
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                       |
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| X                                                                                                                                                               |
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