| **Investments (Narrative) (Details) $ in Millions**                                                                                                                                | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------|-----------------------|-----------------------|
| **Gain (Loss) on Investments**                                                                                                                                                     |                  |                  |                  |
| Largest equity holdings to total equity portfolio                                                                                                                                  | 3.80%                 |                  |                  |
| Largest equity holdings to total portfolio                                                                                                                                         | 1.30%                 |                  |                  |
| Investments, deposit with various states                                                                                                                                           | $ 70                  | $ 74                  |                  |
| Investments in fair value, deposit with various states                                                                                                                             | $ 16,193              | $ 15,114              |                  |
| Impaired securities                                                                                                                                                                | 4                     | 20                    | 6                     |
| Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss                                                                            | $ 11                  | $ 37                  | $ 10                  |
| Fixed Maturities                                                                                                                                                                   |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                                                                                                     |                  |                  |                  |
| Number of securities total unrealized loss in an unrealized loss position for 12 months or more                                                                                    | 32                    | 69                    | 144                   |
| Number of securities below 70 percent of amortized cost                                                                                                                            | 0                     | 5                     | 0                     |
| Equity Securities                                                                                                                                                                  |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                                                                                                     |                  |                  |                  |
| Number of securities total unrealized loss in an unrealized loss position for 12 months or more                                                                                    | 0                     | 0                     | 3                     |
| Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss                                                                            |                  | $ 1                   |                  |
| 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                                                                                                                       | $ 698                 |                  |                  |
| Seven Primary Contributors                                                                                                                                                         |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                                                                                                     |                  |                  |                  |
| Number of securities                                                                                                                                                               | 7                     |                  |                  |
| On Deposit With Various States                                                                                                                                                     |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                                                                                                     |                  |                  |                  |
| Investments in fair value, deposit with various states                                                                                                                             | $ 72                  | $ 77                  |                  |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss, Accumulated in Investments
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLossAccumulatedinInvestments**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
largest equity holdings to total equity portfolio
+ References
No definition available.
+ Details**Name:**cinf_Largestequityholdingstototalequityportfolio**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest equity holdings to total portfolio
+ References
No definition available.
+ Details**Name:**cinf_Largestequityholdingstototalportfolio**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSecurities**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonthsandBelow70PercentofAmortizedCost**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities, Other Than Temporary Impairment Losses
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSecuritiesOtherThanTemporaryImpairmentLosses**Namespace Prefix:**cinf_**Data Type:**xbrli:pureItemType**Balance Type:**na**Period Type:**instant |
| 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=73527792&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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 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
+ Details**Name:**us-gaap_DepositAssets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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 -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(12)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 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 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(6)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 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=d3e24584-111560Reference 6: 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(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 7: 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=65884222&loc=d3e22054-111558Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 9: 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=6505594
+ Details**Name:**us-gaap_InvestmentsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ScheduleOfGainLossOnInvestmentsIncludingMarketableSecuritiesAndInvestmentsHeldAtCostIncomeStatementReportedAmountsSummaryLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**dei_LegalEntityAxis=cinf_ProcterGambleCompanyExxonMobilCorporationAndChevronCorporationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentSecondaryCategorizationAxis=us-gaap_InvestmentsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentSecondaryCategorizationAxis=cinf_Group3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |