| **Investments - Additional Information (Detail)**                                                                       | 3 Months Ended  | 6 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|-----------------|----------------|-----------------|
||
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |           |            |   |   |
| Largest Single Common Stock Holding                                                                                     | $ 206,000,000   |           | $ 206,000,000   |   |   |
| Largest equity holdings to total equity portfolio                                                                       | 3.70%           |           | 3.70%           |   |   |
| Largest equity holdings to total portfolio                                                                              | 1.30%           |           | 1.30%           |   |   |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ (6,000,000)   |           | $ (6,000,000)   |   | $ (11,000,000) |
| 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                                                                  | $ 431,000,000   |           | $ 431,000,000   |   | $ 359,000,000 |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                 | 1               |           | 1               |   |   |
| Provision for Other Credit Losses                                                                                       | $ 0             | $ 0            | $ 0             | $ 0 |   |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months | security                             | 23              |           | 23              |   | 32 |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ (6,000,000)   |           | $ (6,000,000)   |   | $ (11,000,000) |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost | 0               |           | 0               |   | 0 |
| Equity Securities                                                                                                       |            |           |            |   |   |
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |           |            |   |   |
| Unrealized investment gains due to a net gain position                                                                  | $ 2,597,000,000 |           | $ 2,597,000,000 |   | $ 2,348,000,000 |
| Number Of Securities Other Than Temporary Impairment Losses1 | security                                                 | 5               |           | 5               |   |   |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months                                        | 0               |           | 0               |   | 0 |
| Unrealized losses 12 months or more (equity investment less than $500,000)                                              | $ 0             |           | $ 0             |   | $ 0 |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost | 0               |           | 0               |   | 0 |
| Largest unrealized investment gains in common stock portfolio [Member]                                                  |            |           |            |   |   |
| **Gain (Loss) on Investments [Line Items]**                                                                             |            |           |            |   |   |
| Unrealized investment gains due to a net gain position                                                                  | $ 829,000,000   |           | $ 829,000,000   |   |   |
| 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
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.
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| 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.
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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.
+ 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.
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| 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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