| **Supplementary Insurance Information (Details) - USD ($) $ in Millions** | 12 Months Ended |
|---------------------------------------------------------------------------|-----------------|
||
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | $ 637           | $ 616 | $ 578 |
| Gross future policy benefits, losses, claims and expense losses           | 7,728           | 7,265 | 6,957 |
| Gross unearned premium                                                    | 2,307           | 2,201 | 2,082 |
| Other policy claims and benefits payable                                  | 28              | 36 | 25 |
| Earned premiums                                                           | 4,710           | 4,480 | 4,243 |
| Investment income, net of expenses                                        | 539             | 518 | 502 |
| Benefits, claims losses and settlement expense                            | 3,107           | 2,808 | 2,856 |
| Amortization of deferred policy acquisition costs                         | 863             | 829 | 807 |
| Underwriting, acquisition and insurance expenses                          | 602             | 558 | 494 |
| Net written premiums                                                      | 4,582           | 4,363 | 4,146 |
| Commercial Insurance                                                      |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 271             | 264 | 257 |
| Gross future policy benefits, losses, claims and expense losses           | 4,179           | 3,925 | 3,797 |
| Gross unearned premium                                                    | 1,510           | 1,472 | 1,441 |
| Earned premiums                                                           | 3,089           | 2,996 | 2,856 |
| Benefits, claims losses and settlement expense                            | 1,928           | 1,708 | 1,812 |
| Amortization of deferred policy acquisition costs                         | 570             | 552 | 537 |
| Underwriting, acquisition and insurance expenses                          | 412             | 395 | 365 |
| Net written premiums                                                      | 3,122           | 3,025 | 2,922 |
| Personal Insurance                                                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 110             | 103 | 108 |
| Gross future policy benefits, losses, claims and expense losses           | 569             | 498 | 439 |
| Gross unearned premium                                                    | 629             | 593 | 562 |
| Earned premiums                                                           | 1,161           | 1,097 | 1,041 |
| Benefits, claims losses and settlement expense                            | 840             | 789 | 740 |
| Amortization of deferred policy acquisition costs                         | 209             | 210 | 209 |
| Underwriting, acquisition and insurance expenses                          | 128             | 113 | 84 |
| Net written premiums                                                      | 1,198           | 1,128 | 1,068 |
| Excess and Surplus Lines Insurance                                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 16              | 15 | 14 |
| Gross future policy benefits, losses, claims and expense losses           | 241             | 227 | 202 |
| Gross unearned premium                                                    | 93              | 87 | 78 |
| Earned premiums                                                           | 183             | 168 | 148 |
| Benefits, claims losses and settlement expense                            | 68              | 70 | 75 |
| Amortization of deferred policy acquisition costs                         | 31              | 28 | 24 |
| Underwriting, acquisition and insurance expenses                          | 23              | 20 | 19 |
| Net written premiums                                                      | 189             | 175 | 153 |
| Reinsurance assumed and other non segment [Member]                        |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 11              | 6 | 0 |
| Gross future policy benefits, losses, claims and expense losses           | 46              | 10 | 0 |
| Gross unearned premium                                                    | 74              | 48 | 0 |
| Earned premiums                                                           | 49              | 10 | 0 |
| Benefits, claims losses and settlement expense                            | 25              | 5 | 0 |
| Amortization of deferred policy acquisition costs                         | 10              | 2 | 0 |
| Underwriting, acquisition and insurance expenses                          | 6               | 1 | 0 |
| Net written premiums                                                      | 71              | 33 | 0 |
| Property, Liability and Casualty Insurance Product Line [Member]          |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 408             | 388 | 379 |
| Gross future policy benefits, losses, claims and expense losses           | 5,035           | 4,660 | 4,438 |
| Gross unearned premium                                                    | 2,306           | 2,200 | 2,081 |
| Earned premiums                                                           | 4,482           | 4,271 | 4,045 |
| Investment income, net of expenses                                        | 384             | 368 | 358 |
| Benefits, claims losses and settlement expense                            | 2,861           | 2,572 | 2,627 |
| Amortization of deferred policy acquisition costs                         | 820             | 792 | 770 |
| Underwriting, acquisition and insurance expenses                          | 569             | 529 | 468 |
| Net written premiums                                                      | 4,580           | 4,361 | 4,143 |
| Accident Health Insurance Segment [Member]                                |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Net written premiums                                                      | 2               | 2 | 3 |
| Life Insurance Segment                                                    |            |   |   |
| **Supplementary Insurance Information, by Segment**                       |            |   |   |
| Deferred policy acquisition costs                                         | 229             | 228 | 199 |
| Gross future policy benefits, losses, claims and expense losses           | 2,693           | 2,605 | 2,519 |
| Gross unearned premium                                                    | 1               | 1 | 1 |
| Other policy claims and benefits payable                                  | 28              | 36 | 25 |
| Earned premiums                                                           | 228             | 209 | 198 |
| Investment income, net of expenses                                        | 155             | 150 | 144 |
| Benefits, claims losses and settlement expense                            | 246             | 236 | 229 |
| Amortization of deferred policy acquisition costs                         | 43              | 37 | 37 |
| Underwriting, acquisition and insurance expenses                          | $ 33            | $ 29 | $ 26 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of amortization of deferred policy acquisition costs recognized.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationAmortizationOfDeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of insurance benefits, claims, losses and settlement expenses incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationBenefitsClaimsLossesAndSettlementExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationBySegmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net amount of deferred policy acquisition costs capitalized on contracts remaining in force on the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationDeferredPolicyAcquisitionCosts**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of reserves for future policy claims payable and loss expenses to be incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationLiabilityForFuturePolicyBenefitsLossesClaimsAndLossExpenseReserves**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of net investment income earned.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationNetInvestmentIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of other operating expense incurred.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationOtherOperatingExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of claims and benefits payable classified as other.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationOtherPolicyClaimsAndBenefitsPayable**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of premium revenue earned.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationPremiumRevenue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of actual written premiums, including, as applicable, premiums from reinsurance assumed and net of premiums on reinsurance ceded.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationPremiumsWritten**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of premiums not yet earned.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-16) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915
+ Details**Name:**us-gaap_SupplementaryInsuranceInformationUnearnedPremiums**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_SegmentReportingInformationBySecondarySegmentAxis=us-gaap_PropertyAndCasualtyCommercialInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_SegmentReportingInformationBySecondarySegmentAxis=us-gaap_PropertyAndCasualtyPersonalInsuranceProductLineMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_SegmentReportingInformationBySecondarySegmentAxis=cinf_ExcessandSurplusLinesInsuranceMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**cinf_SegmentReportingInformationBySecondarySegmentAxis=cinf_ReinsuranceassumedandothernonsegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_AccidentAndHealthInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementBusinessSegmentsAxis=us-gaap_LifeInsuranceSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |