| **Claims Development - Excess and Surplus Lines (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 ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2012 USD ($) | Dec. 31, 2011 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                                |                  |                  |                  |                  |                  |                  |                  |                  |
| Excess and Surplus Lines Insurance                                                                     |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 658                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | 333                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| All outstanding liabilities before, net of reinsurance                                                 | 1                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Liabilities for loss and ALAE, net of reinsurance                                                      | 326                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| 2011 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 34                                   | $ 34                  | $ 35                  | $ 35                  | $ 36                  | $ 38                  | $ 44                  | $ 47                  | $ 48                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 0                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 1                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 33                                 | 33                    | 34                    | 32                    | 30                    | 27                    | 23                    | 14                    | $ 8                   |
| 2012 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 36                                   | 35                    | 36                    | 37                    | 40                    | 49                    | 56                    | 67                    |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 1                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 1                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 33                                 | 32                    | 31                    | 29                    | 25                    | 19                    | 15                    | $ 9                   |                  |
| 2013 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 41                                   | 41                    | 42                    | 45                    | 54                    | 64                    | 74                    |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 1                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 37                                 | 34                    | 32                    | 27                    | 20                    | 12                    | $ 7                   |                  |                  |
| 2014 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 59                                   | 60                    | 64                    | 75                    | 82                    | 95                    |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 6                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 48                                 | 43                    | 37                    | 27                    | 17                    | $ 9                   |                  |                  |                  |
| 2015 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 65                                   | 67                    | 73                    | 81                    | 96                    |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 8                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 51                                 | 41                    | 29                    | 19                    | $ 8                   |                  |                  |                  |                  |
| 2016 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 82                                   | 84                    | 87                    | 93                    |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 14                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 51                                 | 39                    | 21                    | $ 10                  |                  |                  |                  |                  |                  |
| 2017 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 95                                   | 95                    | 104                   |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 26                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 41                                 | 23                    | $ 11                  |                  |                  |                  |                  |                  |                  |
| 2018 | Excess and Surplus Lines Insurance                                                              |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 109                                  | 116                   |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 44                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 3                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 26                                 | $ 11                  |                  |                  |                  |                  |                  |                  |                  |
| Short-Duration Insurance Contract, Accident Year 2019 | Excess and Surplus Lines Insurance             |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| **Claims Development**                                                                                 |                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Incurred losses and ALAE, net of reinsurance for the years ended December 31,                          | 137                                  |                  |                  |                  |                  |                  |                  |                  |                  |
| Total of incurred but not reported liabilities plus expected development on reported losses            | $ 83                                 |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative number of reported claims | reported_claim                                                  | 2                                    |                  |                  |                  |                  |                  |                  |                  |                  |
| Cumulative paid losses and ALAE, net of reinsurance                                                    | $ 13                                 |                  |                  |                  |                  |                  |                  |                  |                  |

| 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:**us-gaap_StatementBusinessSegmentsAxis=cinf_ExcessandSurplusLinesInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2011Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2012Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2013Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ShortdurationInsuranceContractsAccidentYearAxis=us-gaap_ShortdurationInsuranceContractsAccidentYear2014Member**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:** |