| **Supplementary Information Concerning Property Casualty Insurance Operations (Details) - USD ($) $ in Millions** | 12 Months Ended |
|-------------------------------------------------------------------------------------------------------------------|-----------------|
||
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                           |            |   |   |
| Deferred policy acquisition costs                                                                                 | $ 438           | $ 408 | $ 388 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 5,219           | 5,035 | 4,660 |
| Unearned premiums                                                                                                 | 2,403           | 2,306 | 2,200 |
| Earned premiums                                                                                                   | 4,722           | 4,482 | 4,271 |
| Investment income                                                                                                 | 392             | 384 | 368 |
| Loss and loss expenses incurred related to current accident year                                                  | 3,257           | 3,029 | 2,756 |
| Prior year claims and claims adjustment expense, (favorable) and unfavorable                                      | (119)           | (168) | (184) |
| Amortization of deferred policy acquisition cost                                                                  | 867             | 820 | 792 |
| Paid loss and loss expenses                                                                                       | 2,843           | 2,503 | 2,349 |
| Net written premiums                                                                                              | 4,840           | 4,580 | 4,361 |
| Commercial Insurance                                                                                              |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                           |            |   |   |
| Deferred policy acquisition costs                                                                                 | 284             | 271 | 264 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 4,236           | 4,179 | 3,925 |
| Unearned premiums                                                                                                 | 1,548           | 1,510 | 1,472 |
| Earned premiums                                                                                                   | 3,165           | 3,089 | 2,996 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 2,115           | 2,057 | 1,862 |
| Prior year claims and claims adjustment expense, (favorable) and unfavorable                                      | (73)            | (129) | (154) |
| Amortization of deferred policy acquisition cost                                                                  | 590             | 570 | 552 |
| Paid loss and loss expenses                                                                                       | 1,866           | 1,675 | 1,575 |
| Net written premiums                                                                                              | 3,202           | 3,122 | 3,025 |
| Personal Insurance                                                                                                |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                           |            |   |   |
| Deferred policy acquisition costs                                                                                 | 121             | 110 | 103 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 587             | 569 | 498 |
| Unearned premiums                                                                                                 | 683             | 629 | 593 |
| Earned premiums                                                                                                   | 1,241           | 1,161 | 1,097 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 932             | 844 | 784 |
| Prior year claims and claims adjustment expense, (favorable) and unfavorable                                      | (14)            | (4) | 5 |
| Amortization of deferred policy acquisition cost                                                                  | 225             | 209 | 210 |
| Paid loss and loss expenses                                                                                       | 898             | 771 | 731 |
| Net written premiums                                                                                              | 1,294           | 1,198 | 1,128 |
| Excess and Surplus Lines Insurance                                                                                |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                           |            |   |   |
| Deferred policy acquisition costs                                                                                 | 17              | 16 | 15 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 264             | 241 | 227 |
| Unearned premiums                                                                                                 | 105             | 93 | 87 |
| Earned premiums                                                                                                   | 209             | 183 | 168 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 115             | 102 | 105 |
| Prior year claims and claims adjustment expense, (favorable) and unfavorable                                      | (29)            | (34) | (35) |
| Amortization of deferred policy acquisition cost                                                                  | 35              | 31 | 28 |
| Paid loss and loss expenses                                                                                       | 61              | 55 | 43 |
| Net written premiums                                                                                              | 219             | 189 | 175 |
| Reinsurance assumed and other non segment [Member]                                                                |            |   |   |
| **Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]**                           |            |   |   |
| Deferred policy acquisition costs                                                                                 | 16              | 11 | 6 |
| Reserves for unpaid claims and claim adjustment expenses                                                          | 132             | 46 | 10 |
| Unearned premiums                                                                                                 | 67              | 74 | 48 |
| Earned premiums                                                                                                   | 107             | 49 | 10 |
| Investment income                                                                                                 | 0               | 0 | 0 |
| Loss and loss expenses incurred related to current accident year                                                  | 95              | 26 | 5 |
| Prior year claims and claims adjustment expense, (favorable) and unfavorable                                      | (3)             | (1) | 0 |
| Amortization of deferred policy acquisition cost                                                                  | 17              | 10 | 2 |
| Paid loss and loss expenses                                                                                       | 18              | 2 | 0 |
| Net written premiums                                                                                              | $ 125           | $ 71 | $ 33 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization expense on deferred policy acquisition cost.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersAmortizationOfDeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost related to property-casualty insurance policy written.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersDeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of property-casualty insurance premium revenue earned on insurance policies.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersEarnedPremiums**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| 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_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net investment income earned by property-casualty insurance underwriter.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersNetInvestmentIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Payments of claims and claims adjustment expenses during the period of a property-casualty insurance entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPaidClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of actual premium written by property-casualty insurance underwriter.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPremiumsWritten**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of reserve, policy claim payable and loss expense incurred by property-casualty insurance underwriter.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersReservesForUnpaidClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of property-casualty insurance premium revenue not yet earned on insurance policies.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersUnearnedPremiums**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_ExcessandSurplusLinesInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_ReinsuranceassumedandothernonsegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |