| **Investments - Additional Information (Detail) $ in Millions**                                                         | 3 Months Ended | 9 Months Ended | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-----------------|
||
| **Gain (Loss) on Securities**                                                                                           |           |           |            |   |   |
| Debt Securities, Available-for-sale, Allowance for Credit Loss                                                          | 0              |           | 0               |   |   |
| Equity securities, at fair value | $                                                                                    | $ 7,867        |           | $ 7,867         |   | $ 7,752 |
| Largest Single Common Stock Holding | $                                                                                 | $ 562          |           | $ 562           |   |   |
| Largest equity holdings to total equity portfolio                                                                       | 7.40%          |           | 7.40%           |   |   |
| Largest equity holdings to total portfolio                                                                              | 2.80%          |           | 2.80%           |   |   |
| Number Of Securities Other Than Temporary Impairment Losses1                                                            |           | 2              |            |   |   |
| Fixed maturity investments, Percentage of amortized costs                                                               |           |           | 70.00%          |   | 70.00% |
| Fixed maturities                                                                                                        |           |           |            |   |   |
| **Gain (Loss) on Securities**                                                                                           |           |           |            |   |   |
| Number Of Impaired Securities Intended to be Sold                                                                       | 2              |           | 14              |   |   |
| Number Of Securities Other Than Temporary Impairment Losses1                                                            |           |           |            | 2 |   |
| Number Of Securities Continuous Unrealized Loss Position                                                                | 254            |           | 254             |   |   |
| Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss 3 | $                                | $ 32           |           | $ 32            |   | $ 7 |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months                                        |           |           |            |   | 38 |
| Unrealized losses 12 months or more (equity investment less than $500,000) | $                                          | $ (4)          |           | $ (4)           |   | $ (3) |
| Number Of Securities Continuous Unrealized Loss Position and Below 70 Percent of Amortized Cost                         | 1              |           | 1               |   |   |
| Number Of Securities Continuous Unrealized Loss Position More Than Twelve Months and Below 70 Percent of Amortized Cost |           |           |            |   | 0 |
| Common equities | Equity securities                                                                                     |           |           |            |   |   |
| **Gain (Loss) on Securities**                                                                                           |           |           |            |   |   |
| Equity securities, at fair value | $                                                                                    | $ 7,589        |           | $ 7,589         |   | $ 7,518 |

| 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 loss on securities classified as available-for-sale.
+ References
No definition available.
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| 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.
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| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Largest equity holdings to total portfolio
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of impaired available-for-sale debt securities intended to be sold and written down to fair value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Securities Continuous Unrealized Loss Position
+ 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 Continuous Unrealized Loss Position 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 impaired available-for-sale debt securities not intended to be sold with an allowance for credit losses due to a credit loss
+ References
No definition available.
+ Details**Name:**cinf_NumberofImpairedSecuritieswithAllowanceforCreditLosses**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of equity securities categorized neither as held-to-maturity nor as trading.
+ References
No definition available.
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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                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommonStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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