| **Investments - Additional Information (Detail) $ in Millions**                                                                     | 3 Months Ended | 9 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                                                                         |           |           |            |
| Short-term investments, at fair value (amortized cost: 2015—$75; 2014—$0)                                               | $ 75           | $ 75           | $ 0             |
| Largest equity holdings to total equity portfolio                                                                                   | 3.60%          | 3.60%          |            |
| Largest equity holdings to total portfolio                                                                                          | 1.10%          | 1.10%          |            |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                          | $ 10           | $ 10           | $ 11            |
| Number Of Securities Other Than Temporary Impairment Losses                                                                         |           |           | 6               |
| 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                                                                              | $ 493          | $ 493          | $ 607           |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                             | 2              | 3              |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security                                         | 73             | 73             | 144             |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                          | $ 10           | $ 10           | $ 10            |
| Equity Securities                                                                                                                   |           |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                         |           |           |            |
| Unrealized investment gains due to a net gain position                                                                              | $ 1,613        | $ 1,613        | $ 2,134         |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                             | 1              | 3              |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months                                                    | 0              | 0              | 3               |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                                          | $ 0            | $ 0            | $ 1             |
| Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss, Accumulated in Investments |           |           | $ 1             |
| 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                                                                              | $ 494          | $ 494          |            |
| Common equities                                                                                                                     |           |           |            |
| **Gain (Loss) on Investments [Line Items]**                                                                                         |           |           |            |
| Number of securities                                                                                                                | 7              | 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
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
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 equity holdings to total equity portfolio
+ References
No definition available.
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| 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.
+ Details**Name:**cinf_NumberOfSecuritiesContinuousUnrealizedLossPositionMoreThanTwelveMonths**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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.
+ 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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