| **Employee Retirement Benefits - Additional Information (Detail) (USD $)In Millions, unless otherwise specified** | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]**                    |           |           |   |   |   |
| Matching contributions for 401(k) plan and Top Hat savings plan                                                   | $ 2            |           | $ 2 | $ 7 | $ 7 |
| Pension Contributions                                                                                             |           | 15             |   |   |   |
| Qualified Pension Plans [Member] | Future Period [Member]                                                         |           |           |   |   |   |
| **Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]**                    |           |           |   |   |   |
| Expected contribution by employer for the remainder of the year                                                   | $ 0            |           | $ 0 | $ 0 | $ 0 |

| X                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash issued to participants as the employers' matching contribution to the Company's 401(k) plan.
+ References
No definition available.
+ Details**Name:**cinf_Companys401KEmployerMatchExpense**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**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 |