| **Employee Retirement Benefits - Defined Contribution Plan (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended                                                                                                                                                                             |
|----------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Defined Benefit Plan Disclosure [Line Items]**                                                                     |                                                                                                                                                                                       |   |
| Matching contributions to 401(k) and Top Hat plans                                                                   | $ 4us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount                                                                                                                  | $ 4us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount |
| Qualified Pension Plans [Member]                                                                                     |                                                                                                                                                                                       |   |
| **Defined Benefit Plan Disclosure [Line Items]**                                                                     |                                                                                                                                                                                       |   |
| Expected contributions by employer in the remainder of year                                                          | 0cinf_DefinedBenefitPlansEstimatedFutureEmployerContributionsinCurrentFiscalYear./ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember |   |
| Pension contributions                                                                                                | $ 5us-gaap_PensionContributions/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= us-gaap_PensionPlansDefinedBenefitMember                                                   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of employer's best estimate of contributions expected to be paid to the plan in the current remaining fiscal period.
+ References
No definition available.
+ Details**Name:**cinf_DefinedBenefitPlansEstimatedFutureEmployerContributionsinCurrentFiscalYear.**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlanDisclosureLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of discretionary contributions made by an employer to a defined contribution plan.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash or cash equivalents contributed by the entity to fund its pension plans.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585
+ Details**Name:**us-gaap_PensionContributions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_PensionPlansDefinedBenefitMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |