| **Employee Retirement Benefits (Fair Value Hierarchy Of Assets Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions** | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 |
|-------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | $ 397         | $ 357         | $ 354         |
| Fixed Maturities                                                                                                                                |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 49            | 60            |          |
| Fixed Maturities | Level 1                                                                                                                      |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 31            | 31            |          |
| Fixed Maturities | Level 2                                                                                                                      |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 18            | 29            |          |
| Fixed Maturities | Level 3                                                                                                                      |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | States, Municipalities and Political Subdivisions                                                                            |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 7             | 9             |          |
| Fixed Maturities | States, Municipalities and Political Subdivisions | Level 1                                                                  |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | States, Municipalities and Political Subdivisions | Level 2                                                                  |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 7             | 9             |          |
| Fixed Maturities | States, Municipalities and Political Subdivisions | Level 3                                                                  |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | Domestic Corporate Debt Securities                                                                                           |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 11            | 20            |          |
| Fixed Maturities | Domestic Corporate Debt Securities | Level 1                                                                                 |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | Domestic Corporate Debt Securities | Level 2                                                                                 |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 11            | 20            |          |
| Fixed Maturities | Domestic Corporate Debt Securities | Level 3                                                                                 |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | United States Government                                                                                                     |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 31            | 31            |          |
| Fixed Maturities | United States Government | Level 1                                                                                           |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 31            | 31            |          |
| Fixed Maturities | United States Government | Level 2                                                                                           |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities | United States Government | Level 3                                                                                           |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Equity Securities                                                                                                                               |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 332           | 266           |          |
| Equity Securities | Level 1                                                                                                                     |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 332           | 266           |          |
| Equity Securities | Level 2                                                                                                                     |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Equity Securities | Level 3                                                                                                                     |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 0             | 0             |          |
| Fixed Maturities And Equity Securities                                                                                                          |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 381           | 326           |          |
| Fixed Maturities And Equity Securities | Level 1                                                                                                |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 363           | 297           |          |
| Fixed Maturities And Equity Securities | Level 2                                                                                                |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | 18            | 29            |          |
| Fixed Maturities And Equity Securities | Level 3                                                                                                |          |          |          |
| **Defined Benefit Plan Fair Value Disclosure**                                                                                                  |          |          |          |
| Pension plan assets at fair value                                                                                                               | $ 0           | $ 0           |          |

| 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_DefinedBenefitPlanDisclosureLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920Reference 7: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920
+ Details**Name:**us-gaap_DefinedBenefitPlanFairValueOfPlanAssets**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_DomesticCorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=cinf_FixedMaturitiesAndEquitySecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |