| **Investments (Narrative) (Details) $ in Millions**                                                     | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) |
|---------------------------------------------------------------------------------------------------------|-----------------------|-----------------------|-----------------------|
| **Gain (Loss) on Investments**                                                                          |                  |                  |                  |
| Largest single common stock holding                                                                     | $ 255                 |                  |                  |
| Largest equity holdings to total equity portfolio                                                       | 4.40%                 |                  |                  |
| Largest equity holdings to total portfolio                                                              | 1.50%                 |                  |                  |
| Investments in fair value, deposit with various states                                                  | $ 17,434              | $ 17,774              |                  |
| Collateral at fair value                                                                                | $ 75                  | $ 57                  |                  |
| Impaired securities                                                                                     | 1                     | 6                     | 4                     |
| Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate loss | $ 58                  | $ 18                  | $ 11                  |
| Fixed Maturities                                                                                        |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                          |                  |                  |                  |
| Number of securities total unrealized loss in an unrealized loss position for 12 months or more         | 400                   | 249                   | 32                    |
| Number of securities below 70 percent of amortized cost                                                 | 0                     | 0                     | 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                     |
| On Deposit With Various States                                                                          |                  |                  |                  |
| **Gain (Loss) on Investments**                                                                          |                  |                  |                  |
| Investments in fair value, deposit with various states                                                  | $ 112                 | $ 101                 |                  |

| 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 Single Common Stock Holding
+ References
No definition available.
+ Details**Name:**cinf_LargestSingleCommonStockHolding**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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 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
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 -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 3: 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=82895926&loc=d3e24584-111560Reference 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 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 6: 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=75030427&loc=d3e22054-111558Reference 7: 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=d3e572229-122910
+ 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524
+ Details**Name:**us-gaap_SecuritiesHeldAsCollateralAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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:**us-gaap_InvestmentSecondaryCategorizationAxis=cinf_Group3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |