| **Claims Development - Personal Auto (Details) reported_claim in Thousands, $ in Millions** | Dec. 31, 2019 USD ($) reported_claim | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) |
|---------------------------------------------------------------------------------------------|--------------------------------------|-----------------------|-----------------------|-----------------------|-----------------------|
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | $ 2,739                              |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                           | 5,799                                |                  |                  |                  |                  |
| Personal Auto | Personal Insurance                                                          |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 1,933                                |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | 1,659                                |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                      | 6                                    |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                           | 280                                  |                  |                  |                  |                  |
| 2015 | Personal Auto | Personal Insurance                                                   |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 361                                  | $ 360                 | $ 356                 | $ 356                 | $ 343                 |
| Total of incurred but not reported liabilities plus expected development on reported losses | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                       | 108                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | $ 354                                | 346                   | 325                   | 292                   | $ 229                 |
| 2016 | Personal Auto | Personal Insurance                                                   |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 384                                  | 386                   | 384                   | 383                   |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                       | 110                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | $ 370                                | 351                   | 316                   | $ 243                 |                  |
| 2017 | Personal Auto | Personal Insurance                                                   |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 391                                  | 394                   | 412                   |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses | $ 0                                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                       | 109                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | $ 358                                | 324                   | $ 256                 |                  |                  |
| 2018 | Personal Auto | Personal Insurance                                                   |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 398                                  | 424                   |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses | $ 18                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                       | 111                                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | $ 327                                | $ 262                 |                  |                  |                  |
| Short-Duration Insurance Contract, Accident Year 2019 | Personal Auto | Personal Insurance  |                                 |                  |                  |                  |                  |
| **Claims Development**                                                                      |                                 |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,               | 399                                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses | $ 54                                 |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                       | 96                                   |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                         | $ 250                                |                  |                  |                  |                  |

| 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_ClaimsDevelopmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after reinsurance, of paid claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsCumulativePaidClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred-but-not-reported (IBNR) liabilities plus expected development on reported claims, for claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4E -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671321-158438Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671317-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredButNotReportedIbnrClaimsLiabilityNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsIncurredClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671315-158438Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Undiscounted amount, after reinsurance, of the liability for unpaid claims and allocated claim adjustment expense for short-duration insurance contracts for accident years not separately presented in claim development information. Excludes unallocated claim adjustment expense.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4B -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671311-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsLiabilityForUnpaidClaimsAndAllocatedClaimAdjustmentExpenseNetNotSeparatelyPresented**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cumulative number of reported claims for short-duration insurance contracts.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4D -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=116884468&loc=SL65671317-158438
+ Details**Name:**us-gaap_ShortdurationInsuranceContractsNumberOfReportedClaims**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_PersonalAutoMember**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_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2015Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2016Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2017Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractsAccidentYear2018Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortDurationInsuranceContractAccidentYear2019Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |