| **Investments - Additional Information (Detail) (USD $)**                             | 3 Months Ended | 12 Months Ended |
|---------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                           |           |            |
| Fixed maturities, fair value of hybrid securities                                     | $ 17,000,000   | $ 18,000,000    |
| Unrealized losses 12 months or more (equity investment less than $500,000)            | 19,000,000     | 5,000,000       |
| Fixed maturity investments, Percentage of amortized costs                             | 70.00%         | 70.00%          |
| Fixed Maturities                                                                      |           |            |
| **Gain (Loss) on Investments [Line Items]**                                           |           |            |
| Number Of Securities Other Than Temporary Impairment Losses                           | 1              | 7               |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months      | 99             | 40              |
| Unrealized investment gains due to a net gain position                                | 613,000,000    | 558,000,000     |
| Unrealized losses 12 months or more (equity investment less than $500,000)            | 19,000,000     | 5,000,000       |
| Equity Securities                                                                     |           |            |
| **Gain (Loss) on Investments [Line Items]**                                           |           |            |
| Number Of Securities Other Than Temporary Impairment Losses                           | 2              |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months      | 1              |            |
| Unrealized investment gains due to a net gain position                                | 1,888,000,000  | 1,856,000,000   |
| Unrealized losses 12 months or more (equity investment less than $500,000)            | 500,000,000    | 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                                | $ 431,000,000  |            |
| Common Stock                                                                          |           |            |
| **Gain (Loss) on Investments [Line Items]**                                           |           |            |
| Number of securities                                                                  | 5              |            |

| 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.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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
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:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of hybrid instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13433-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13495-108611Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 15 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6437212&loc=d3e50806-113969
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| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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