| **Reserves in Addition to Account Balance, Based on Expected No-Lapse Guarantee Benefits and Expected Policy Assessments (Detail) - USD ($) $ in Millions** | Sep. 30, 2016 | Dec. 31, 2015 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | $ 2,641       | $ 2,583       |
| Life policy reserves:                                                                                                                                       |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 1,036         | 987           |
| Life policy reserves: | Ordinary/traditional life                                                                                                           |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 991           | 943           |
| Life policy reserves: | Other                                                                                                                               |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 45            | 44            |
| Investment contract reserves:                                                                                                                               |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 1,605         | 1,596         |
| Investment contract reserves: | Deferred annuities                                                                                                          |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 864           | 860           |
| Investment contract reserves: | Universal life                                                                                                              |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 565           | 558           |
| Investment contract reserves: | Structured settlements                                                                                                      |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | 170           | 174           |
| Investment contract reserves: | Other                                                                                                                       |          |          |
| **Liability for Future Policy Benefit, by Product Segment [Line Items]**                                                                                    |          |          |
| Life policy and investment contract reserves                                                                                                                | $ 6           | $ 4           |

| 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://www.xbrl.org/2003/role/presentationRef -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=6879938&loc=d3e572229-122910Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 25 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28366191&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:**us-gaap_ProductOrServiceAxis=cinf_IndividualOrdinaryLifeInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_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:**us-gaap_ProductOrServiceAxis=cinf_DeferredAnnuitiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=us-gaap_UniversalLifeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=cinf_StructuredSettlementsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ProductOrServiceAxis=cinf_OtherInvestmentContractReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |