| **Reserves in Addition to Account Balance, Based on Expected No-Lapse Guarantee Benefits and Expected Policy Assessments (Detail) - USD ($) $ in Millions** | Mar. 31, 2020 | Dec. 31, 2019 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | $ 2,861       | $ 2,835       |
| Life policy reserves:                                                                                                                                       |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 1,295         | 1,276         |
| Life policy reserves: | Ordinary/traditional life                                                                                                           |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 1,245         | 1,226         |
| Life policy reserves: | Other                                                                                                                               |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 50            | 50            |
| Investment contract reserves:                                                                                                                               |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 1,566         | 1,559         |
| Investment contract reserves: | Deferred annuities                                                                                                          |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 754           | 760           |
| Investment contract reserves: | Universal life                                                                                                              |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 653           | 640           |
| Investment contract reserves: | Structured Settlement Annuity                                                                                               |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | 151           | 151           |
| Investment contract reserves: | Other                                                                                                                       |          |          |
| **Liability for Future Policy Benefit, by Product Segment**                                                                                                 |          |          |
| Life policy and investment contract reserves                                                                                                                | $ 8           | $ 8           |

| 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.
+ Details**Name:**us-gaap_LiabilityForFuturePolicyBenefitByProductSegmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accrued obligation to policyholders that relates to insured events under long-term life insurance contracts.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(a)(1)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 25 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=117419144&loc=d3e13145-158430
+ Details**Name:**us-gaap_LiabilityForFuturePolicyBenefitsLife**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_EquitySecuritiesByIndustryAxis=cinf_LifePolicyReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_IndividualOrdinaryLifeInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_OtherLifePolicyReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_EquitySecuritiesByIndustryAxis=cinf_InvestmentcontractreservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_DeferredAnnuitiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=us-gaap_UniversalLifeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=us-gaap_StructuredSettlementAnnuityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_OtherInvestmentContractReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |