| **Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)In Millions, unless otherwise specified**            | 12 Months Ended                                                                                                                    |
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| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Number of operating subsidaries                                                                                                | 2cinf_NumberOfSubsidiaries                                                                                                         |   |   |   |
| Number of independent insurance agencies                                                                                       | 1,466cinf_IndependentInsuranceAgenciesAndBrokerages                                                                                |   |   |   |
| Number of reporting locations                                                                                                  | 1,884cinf_NumberOfLocations                                                                                                        |   |   |   |
| Number of states in which entity operates                                                                                      | 39us-gaap_NumberOfStatesInWhichEntityOperates                                                                                      |   |   |   |
| Defined Benefit Plan Qualifying Age                                                                                            |                                                                                                                               |   |   | 40 years |
| Current fair value of the BOLI invested assets and cash exceeded the current fair value of the contract holders' account value | $ 47us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentFairValue                                              |   |   |   |
| Other invested assets                                                                                                          | 68us-gaap_OtherInvestments                                                                                                         | 68us-gaap_OtherInvestments |   |   |
| Depreciation expense                                                                                                           | 37us-gaap_Depreciation                                                                                                             | 37us-gaap_Depreciation | 37us-gaap_Depreciation |   |
| Land, building and equipment impairments                                                                                       | 0us-gaap_AssetImpairmentCharges                                                                                                    | 0us-gaap_AssetImpairmentCharges | 0us-gaap_AssetImpairmentCharges |   |
| Minimum                                                                                                                        |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Estimated useful lives                                                                                                         | 3 years                                                                                                                            |   |   |   |
| Maximum                                                                                                                        |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Estimated useful lives                                                                                                         | 39 years 6 months                                                                                                                  |   |   |   |
| Life Policy Loans                                                                                                              |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Other invested assets                                                                                                          | 31us-gaap_OtherInvestments/ us-gaap_InvestmentTypeAxis= cinf_LifePolicyLoansMember                                                 |   |   |   |
| Venture Capital Funds                                                                                                          |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Other invested assets                                                                                                          | $ 37us-gaap_OtherInvestments/ us-gaap_InvestmentTypeAxis= us-gaap_VentureCapitalFundsMember                                        |   |   |   |
| 10 Largest States                                                                                                              |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 62.80%cinf_PercentageOfPremiums/ us-gaap_StatementGeographicalAxis= cinf_StateMember                                               | 63.80%cinf_PercentageOfPremiums/ us-gaap_StatementGeographicalAxis= cinf_StateMember |   |   |
| Ohio, Largest State                                                                                                            |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 17.70%cinf_PercentageOfPremiums/ us-gaap_StatementGeographicalAxis= stpr_OH                                                        | 18.50%cinf_PercentageOfPremiums/ us-gaap_StatementGeographicalAxis= stpr_OH |   |   |
| Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina | Minimum                                                |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 5.00%cinf_PercentageOfPremiums/ us-gaap_RangeAxis= us-gaap_MinimumMember/ us-gaap_StatementGeographicalAxis= cinf_OtherStateMember |   |   |   |
| Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina | Maximum                                                |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 8.00%cinf_PercentageOfPremiums/ us-gaap_RangeAxis= us-gaap_MaximumMember/ us-gaap_StatementGeographicalAxis= cinf_OtherStateMember |   |   |   |
| Largest Single Agency Relationship                                                                                             |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 0.80%cinf_PercentageOfPremiums/ us-gaap_MajorCustomersAxis= cinf_LargestCustomerMember                                             |   |   |   |
| All Other | Maximum                                                                                                            |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Generated percentage of total earned premiums                                                                                  | 2.10%cinf_PercentageOfPremiums/ us-gaap_MajorCustomersAxis= cinf_AllOtherMember/ us-gaap_RangeAxis= us-gaap_MaximumMember          |   |   |   |
| Stock Options                                                                                                                  |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Awards vesting period                                                                                                          | 3 years                                                                                                                            |   |   |   |
| Weighted-average remaining contractual life for options expected to vest                                                       | 10 years                                                                                                                           |   |   |   |
| Restricted Stock Units                                                                                                         |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Awards vesting period                                                                                                          | 3 years                                                                                                                            |   |   |   |
| Property Casualty Insurance Segment                                                                                            |                                                                                                                               |   |   |   |
| **Significant Accounting Policies**                                                                                            |                                                                                                                               |   |   |   |
| Number of operating subsidaries                                                                                                | 2cinf_NumberOfSubsidiaries/ us-gaap_StatementBusinessSegmentsAxis= cinf_PropertyCasualtyInsuranceSegmentMember                     |   |   |   |

| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Age at which participants could elect to remain in the defined benefit plan as of August 31, 2008.
+ References
No definition available.
+ Details**Name:**cinf_DefinedBenefitPlanQualifyingAge**Namespace Prefix:**cinf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Independent Insurance Agencies and Brokerages
+ References
No definition available.
+ Details**Name:**cinf_IndependentInsuranceAgenciesAndBrokerages**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Locations
+ References
No definition available.
+ Details**Name:**cinf_NumberOfLocations**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Subsidiaries
+ References
No definition available.
+ Details**Name:**cinf_NumberOfSubsidiaries**Namespace Prefix:**cinf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of earned premiums.
+ References
No definition available.
+ Details**Name:**cinf_PercentageOfPremiums**Namespace Prefix:**cinf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_SignificantAccountingPoliciesLineItems**Namespace Prefix:**cinf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.
+ References
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+ Details**Name:**us-gaap_AssetImpairmentCharges**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.
+ References
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+ Details**Name:**us-gaap_Depreciation**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of states the entity operates in as of the balance sheet date.
+ References
No definition available.
+ Details**Name:**us-gaap_NumberOfStatesInWhichEntityOperates**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other investments not otherwise specified in the taxonomy.
+ References
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+ Details**Name:**us-gaap_OtherInvestments**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.
+ References
No definition available.
+ Details**Name:**us-gaap_PropertyPlantAndEquipmentUsefulLife**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value amount of the specified investment category of assets maintained in separate accounts as of the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
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+ Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average remaining contractual term of exercisable stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
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+ Details**Name:**us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageRemainingContractualTerm2**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAxis=us-gaap_EmployeeStockOptionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_ScheduleOfShareBasedCompensationArrangementByShareBasedPaymentAwardAxis=us-gaap_RestrictedStockUnitsRSUMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=cinf_LifePolicyLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_InvestmentTypeAxis=us-gaap_VentureCapitalFundsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorCustomersAxis=cinf_AllOtherMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorCustomersAxis=cinf_LargestCustomerMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_PropertyCasualtyInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementGeographicalAxis=cinf_OtherStateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementGeographicalAxis=cinf_StateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementGeographicalAxis=stpr_OH**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |