| **Investments - Fair Values and Unrealized Losses by Investment Category and by Duration of Securities' Continuous Unrealized Loss Position (Detail) - Fixed maturities - USD ($) $ in Millions** | Jun. 30, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | $ 104         | $ 3,031       |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 2             | 70            |
| 12 months or more Fair Value                                                                                                                                                                      | 441           | 1,149         |
| 12 months or more Unrealized Losses                                                                                                                                                               | 12            | 58            |
| Total Fair Value                                                                                                                                                                                  | 545           | 4,180         |
| Total Unrealized Losses                                                                                                                                                                           | 14            | 128           |
| Corporate                                                                                                                                                                                         |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 78            | 2,082         |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 2             | 51            |
| 12 months or more Fair Value                                                                                                                                                                      | 294           | 501           |
| 12 months or more Unrealized Losses                                                                                                                                                               | 11            | 36            |
| Total Fair Value                                                                                                                                                                                  | 372           | 2,583         |
| Total Unrealized Losses                                                                                                                                                                           | 13            | 87            |
| States, municipalities and political subdivisions                                                                                                                                                 |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 13            | 823           |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 0             | 18            |
| 12 months or more Fair Value                                                                                                                                                                      | 21            | 340           |
| 12 months or more Unrealized Losses                                                                                                                                                               | 0             | 13            |
| Total Fair Value                                                                                                                                                                                  | 34            | 1,163         |
| Total Unrealized Losses                                                                                                                                                                           | 0             | 31            |
| Commercial mortgage-backed                                                                                                                                                                        |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 0             | 77            |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 0             | 0             |
| 12 months or more Fair Value                                                                                                                                                                      | 5             | 64            |
| 12 months or more Unrealized Losses                                                                                                                                                               | 0             | 2             |
| Total Fair Value                                                                                                                                                                                  | 5             | 141           |
| Total Unrealized Losses                                                                                                                                                                           | 0             | 2             |
| Government-sponsored enterprises                                                                                                                                                                  |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 7             | 49            |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 0             | 1             |
| 12 months or more Fair Value                                                                                                                                                                      | 117           | 211           |
| 12 months or more Unrealized Losses                                                                                                                                                               | 1             | 6             |
| Total Fair Value                                                                                                                                                                                  | 124           | 260           |
| Total Unrealized Losses                                                                                                                                                                           | 1             | 7             |
| United States government                                                                                                                                                                          |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 5             | 0             |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 0             | 0             |
| 12 months or more Fair Value                                                                                                                                                                      | 4             | 33            |
| 12 months or more Unrealized Losses                                                                                                                                                               | 0             | 1             |
| Total Fair Value                                                                                                                                                                                  | 9             | 33            |
| Total Unrealized Losses                                                                                                                                                                           | 0             | $ 1           |
| Foreign government                                                                                                                                                                                |          |          |
| **Investments, Unrealized Loss Position**                                                                                                                                                         |          |          |
| Less than 12 months Fair Value                                                                                                                                                                    | 1             |          |
| Less than 12 months Unrealized Losses                                                                                                                                                             | 0             |          |
| 12 months or more Fair Value                                                                                                                                                                      | 0             |          |
| 12 months or more Unrealized Losses                                                                                                                                                               | 0             |          |
| Total Fair Value                                                                                                                                                                                  | 1             |          |
| Total Unrealized Losses                                                                                                                                                                           | $ 0           |          |

| 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 loss on securities classified as available-for-sale.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months.
+ References
No definition available.
+ Details**Name:**cinf_AvailableforsaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss3**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investments, Unrealized Loss Position [Line Items]
+ References
No definition available.
+ Details**Name:**cinf_InvestmentsUnrealizedLossPositionLineItems**Namespace Prefix:**cinf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for twelve months or longer.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**invest_InvestmentSecuritiesClassAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |