| **Investments - Additional Information (Detail) $ in Millions**                                                                    | 3 Months Ended | 12 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                                                                        |           |            |
| Largest Single Common Stock Holding                                                                                                | $ 153          |            |
| Largest equity holdings to total equity portfolio                                                                                  | 3.20%          |            |
| Largest equity holdings to total portfolio                                                                                         | 1.00%          |            |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                         | $ 24           | $ 37            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost | security | 2              | 5               |
| Number Of Securities Other Than Temporary Impairment Losses                                                                        |           | 20              |
| Fixed maturity investments, Percentage of amortized costs                                                                          | 70.00%         | 70.00%          |
| Fixed Maturities                                                                                                                   |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                        |           |            |
| Unrealized investment gains due to a net gain position                                                                             | $ 510          | $ 431           |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                            | 2              |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security                                        | 44             | 69              |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                         | $ 24           | $ 37            |
| Equity Securities                                                                                                                  |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                        |           |            |
| Unrealized investment gains due to a net gain position                                                                             | $ 1,987        | $ 1,819         |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                            | 0              |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months                                                   | 0              | 0               |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                         | $ 0            | $ 0             |
| Procter And Gamble Company, Exxon Mobile Corporation And Chevron Corporation [Member]                                              |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                        |           |            |
| Unrealized investment gains due to a net gain position                                                                             | $ 597          |            |
| Common equities                                                                                                                    |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                        |           |            |
| Number of securities                                                                                                               | 7              |            |

| X                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesGrossUnrealizedGain1**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fixed maturity investments, Percentage of amortized costs.
+ References
No definition available.
+ Details**Name:**cinf_FixedMaturityInvestmentsPercentageOfAmortizedCosts**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest Single Common Stock Holding
+ References
No definition available.
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| 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.
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| 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.
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| 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
Number Of Securities Other Than Temporary Impairment Losses
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSecuritiesOtherThanTemporaryImpairmentLosses1**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| 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                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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