| **Investments (Analysis Of Cost Or Amortized Cost Gross Unrealized Gains And Losses And Fair Value For Investments) (Details) - Fixed Maturities - USD ($) $ in Millions** | Dec. 31, 2019 | Dec. 31, 2018 |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | $ 11,108      | $ 10,643      |
| Gross unrealized gains                                                                                                                                                     | 597           | 174           |
| Gross unrealized losses                                                                                                                                                    | 7             | 128           |
| Fair value                                                                                                                                                                 | 11,698        | 10,689        |
| Corporate Securities                                                                                                                                                       |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 6,074         | 5,712         |
| Gross unrealized gains                                                                                                                                                     | 332           | 85            |
| Gross unrealized losses                                                                                                                                                    | 5             | 87            |
| Fair value                                                                                                                                                                 | 6,401         | 5,710         |
| States, Municipalities and Political Subdivisions                                                                                                                          |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 4,477         | 4,251         |
| Gross unrealized gains                                                                                                                                                     | 252           | 84            |
| Gross unrealized losses                                                                                                                                                    | 1             | 31            |
| Fair value                                                                                                                                                                 | 4,728         | 4,304         |
| Commercial Mortgage Backed Securities                                                                                                                                      |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 290           | 287           |
| Gross unrealized gains                                                                                                                                                     | 11            | 3             |
| Gross unrealized losses                                                                                                                                                    | 0             | 2             |
| Fair value                                                                                                                                                                 | 301           | 288           |
| Government-Sponsored Enterprises                                                                                                                                           |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 137           | 316           |
| Gross unrealized gains                                                                                                                                                     | 0             | 1             |
| Gross unrealized losses                                                                                                                                                    | 1             | 7             |
| Fair value                                                                                                                                                                 | 136           | 310           |
| United States Government                                                                                                                                                   |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 102           | 67            |
| Gross unrealized gains                                                                                                                                                     | 2             | 1             |
| Gross unrealized losses                                                                                                                                                    | 0             | 1             |
| Fair value                                                                                                                                                                 | 104           | 67            |
| Foreign Government                                                                                                                                                         |          |          |
| **Schedule of Available-for-sale Securities**                                                                                                                              |          |          |
| Cost or amortized cost                                                                                                                                                     | 28            | 10            |
| Gross unrealized gains                                                                                                                                                     | 0             | 0             |
| Gross unrealized losses                                                                                                                                                    | 0             | 0             |
| Fair value                                                                                                                                                                 | $ 28          | $ 10          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ References
No definition available.
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| 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_ScheduleOfAvailableForSaleSecuritiesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_DebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CorporateBondSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancialInstrumentAxis=cinf_ForeignGovernmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |