| **Deferred Acquisition Costs - Deferred Policy Acquisition Costs and Asset Reconciliation (Detail) - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                   |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                 | $ 1,045        | $ 924          | $ 905 | $ 805 |
| Capitalized deferred policy acquisition costs                                                                                | 341            | 302            | 1,110 | 968 |
| Amortized deferred policy acquisition costs                                                                                  | (354)          | (312)          | (993) | (884) |
| Shadow deferred policy acquisition costs                                                                                     | 4              | 1              | 14 | 26 |
| Deferred policy acquisition costs asset, end of period                                                                       | 1,036          | 915            | 1,036 | 915 |
| Consolidated Property And Casualty Insurance                                                                                 |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                   |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                 | 723            | 630            | 602 | 542 |
| Capitalized deferred policy acquisition costs                                                                                | 325            | 287            | 1,064 | 924 |
| Amortized deferred policy acquisition costs                                                                                  | (342)          | (300)          | (960) | (849) |
| Deferred policy acquisition costs asset, end of period                                                                       | 706            | 617            | 706 | 617 |
| Life Insurance Segment                                                                                                       |           |           |   |   |
| **Movement Analysis of Deferred Policy Acquisition Costs**                                                                   |           |           |   |   |
| Deferred policy acquisition costs asset, beginning of period                                                                 | 322            | 294            | 303 | 263 |
| Capitalized deferred policy acquisition costs                                                                                | 16             | 15             | 46 | 44 |
| Amortized deferred policy acquisition costs                                                                                  | (12)           | (12)           | (33) | (35) |
| Shadow deferred policy acquisition costs                                                                                     | 4              | 1              | 14 | 26 |
| Deferred policy acquisition costs asset, end of period                                                                       | $ 330          | $ 298          | $ 330 | $ 298 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization expense (reversal of expense) for deferred policy acquisition costs.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 55 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124504731&loc=d3e11522-158419Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (d) -URI https://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(1) -URI https://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(7)(a)) -URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2B -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124505477&loc=SL117422543-158416Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other increase (decrease) to amortization expense of deferred policy acquisition cost (DAC).
+ References
No definition available.
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostAmortizationExpenseOther**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost capitalized on contract remaining in force.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2B -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=124505477&loc=SL117422543-158416Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2B -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124505477&loc=SL117422543-158416Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 825 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=124506110&loc=d3e32546-158582Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(1) -URI https://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(7)) -URI https://asc.fasb.org/extlink&oid=126734703&loc=d3e572229-122910Reference 6: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 55 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124504731&loc=d3e11522-158419
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of deferred policy acquisition cost capitalized.
+ References
Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 2B -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124505477&loc=SL117422543-158416Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 65 -Paragraph 2 -Subparagraph (g)(1) -URI https://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641Reference 3: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 55 -Paragraph 2 -URI https://asc.fasb.org/extlink&oid=124504731&loc=d3e11522-158419
+ Details**Name:**us-gaap_DeferredPolicyAcquisitionCostsAdditions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
+ Details**Name:**us-gaap_MovementAnalysisOfDeferredPolicyAcquisitionCostsRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_ConsolidatedPropertyAndCasualtyInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=cinf_LifeInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |