| **Investments - Additional Information (Detail)**                                                                       | 3 Months Ended  | 9 Months Ended  | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |            |            |
| Largest Single Common Stock Holding                                                                                     | $ 158,000,000   | $ 158,000,000   |            |
| Largest equity holdings to total equity portfolio                                                                       | 3.10%           | 3.10%           |            |
| Largest equity holdings to total portfolio                                                                              | 1.00%           | 1.00%           |            |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ 65,000,000    | $ 65,000,000    | $ 37,000,000    |
| 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                                                                  | $ 615,000,000   | $ 615,000,000   | $ 431,000,000   |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                 | 0               | 4               |            |
| Provision for Other Credit Losses                                                                                       | $ 0             |            |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security                             | 36              | 36              | 69              |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ 10,000,000    | $ 10,000,000    | $ 37,000,000    |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost | 1               | 1               | 5               |
| Equity Securities                                                                                                       |            |            |            |
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |            |            |
| Unrealized investment gains due to a net gain position                                                                  | $ 2,196,000,000 | $ 2,196,000,000 | $ 1,819,000,000 |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                 | 0               | 0               |            |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months                                        | 3               | 3               | 0               |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ 55,000,000    | $ 55,000,000    | $ 0             |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost | 1               | 1               |            |
| Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value below 70 percent    | $ 36,000,000    | $ 36,000,000    |            |
| Largest unrealized investment gains in common stock portfolio [Member]                                                  |            |            |            |
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |            |            |
| Unrealized investment gains due to a net gain position                                                                  | $ 649,000,000   | $ 649,000,000   |            |
| 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, Fair Value below 70 percent
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPosition12MonthsorLongerFairValuebelow70percent**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.
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| 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.
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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.
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| 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.
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| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities, Other Than Temporary Impairment Losses
+ References
No definition available.
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| 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
Amount of expense related to credit loss from transactions other than loan and lease transactions.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(11)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585
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| 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.
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| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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