| **Property Casualty Loss and Loss Expenses - Additional Information (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |         |         |         |         |
|--------------------------------------------------------------------------------------------------------|----------------|----------------|---------|---------|---------|---------|
||
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                     | $ 8,045        | $ 7,226        | $ 8,045 | $ 7,226 | $ 7,603 | $ 7,229 | $ 6,955 | $ 6,677 |
| Prior accident years                                                                                   | (43)           | (102)          | (143)   | (331)   |    |    |   |   |
| Life and Health Loss Reserves                                                                          |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Reserve for loss and loss expenses                                                                     | 68             | 66             | 68      | 66      |    |    |   |   |
| Commercial Insurance                                                                                   |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | (4)            | (107)          | (51)    | (276)   |    |    |   |   |
| Commercial Insurance | Commercial Casualty Line                                                        |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | 23             | (52)           | 25      | (85)    |    |    |   |   |
| Commercial Insurance | Workers' Compensation Line                                                      |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | (16)           |           | (43)    | (59)    |    |    |   |   |
| Commercial Insurance | Commercial Auto Line                                                            |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | 16             |           | 15      | (44)    |    |    |   |   |
| Commercial Insurance | Commercial Property Line                                                        |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | (24)           | (34)           | (36)    | (68)    |    |    |   |   |
| Personal Insurance                                                                                     |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | (8)            | (3)            | (56)    | (35)    |    |    |   |   |
| Personal Insurance | Personal Auto                                                                     |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   |           |           |    | (24)    |    |    |   |   |
| Personal Insurance | Homeowner                                                                         |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   |           |           | (51)    |    |    |    |   |   |
| Excess and Surplus Lines Insurance                                                                     |           |           |    |    |    |    |   |   |
| **SEC Schedule, 12-18, Supplemental Information, Property-Casualty Insurance Underwriters**            |           |           |    |    |    |    |   |   |
| Prior accident years                                                                                   | $ (7)          | $ 3            | $ (13)  | $ 6     |    |    |   |   |

| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**srt_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersLineItems**Namespace Prefix:**srt_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
+ Details**Name:**us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| 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/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437
+ Details**Name:**us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_LifeandHealthLossReservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_CommercialLinesInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_CommercialCasualtySegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_WorkersCompensationMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_CommercialAutoSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_CommercialPropertySegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_PersonalLinesInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_PersonalAutoMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=cinf_HomeownerMember**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:** |