| **Changes in Policyholders' Account Balances (Details) - Investment contract reserves: - USD ($) $ in Millions** | 3 Months Ended | 9 Months Ended |
|------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| Deferred Fixed Annuity                                                                                           |           |           |   |   |
| **Policyholder Account Balance [Roll Forward]**                                                                  |           |           |   |   |
| Balance, beginning of period                                                                                     | $ 696          | $ 755          | $ 734 | $ 763 |
| Premiums received                                                                                                | 13             | 12             | 38 | 23 |
| Policy charges                                                                                                   | 0              | 0              | 0 | 0 |
| Surrenders and withdrawals                                                                                       | (32)           | (17)           | (100) | (37) |
| Benefit payments                                                                                                 | (3)            | (3)            | (9) | (13) |
| Interest credited                                                                                                | 6              | 5              | 17 | 16 |
| Balance, end of period                                                                                           | $ 680          | $ 752          | $ 680 | $ 752 |
| Weighted average crediting rate                                                                                  | 3.45%          | 3.10%          | 3.45% | 3.10% |
| Net amount at risk                                                                                               | $ 0            | $ 0            | $ 0 | $ 0 |
| Cash surrender value                                                                                             | 675            | 748            | 675 | 748 |
| Universal life                                                                                                   |           |           |   |   |
| **Policyholder Account Balance [Roll Forward]**                                                                  |           |           |   |   |
| Balance, beginning of period                                                                                     | 456            | 457            | 457 | 454 |
| Premiums received                                                                                                | 8              | 8              | 28 | 29 |
| Policy charges                                                                                                   | (9)            | (10)           | (29) | (29) |
| Surrenders and withdrawals                                                                                       | (2)            | (4)            | (9) | (9) |
| Benefit payments                                                                                                 | (1)            | (1)            | (5) | (4) |
| Interest credited                                                                                                | 4              | 5              | 14 | 14 |
| Balance, end of period                                                                                           | $ 456          | $ 455          | $ 456 | $ 455 |
| Weighted average crediting rate                                                                                  | 4.29%          | 4.24%          | 4.29% | 4.24% |
| Net amount at risk                                                                                               | $ 3,989        | $ 4,108        | $ 3,989 | $ 4,108 |
| Cash surrender value                                                                                             | $ 424          | $ 421          | $ 424 | $ 421 |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash to be paid to policyholder upon termination and surrender of long-duration contract issued by insurance entity.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (b)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_CashSurrenderValueDuePolicyholdersAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest expense for allocation to policyholder account balance.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479868/944-20-45-2Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 13J -Subparagraph (g) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-13JReference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(6)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_InterestCreditedToPolicyholdersAccountBalances**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash outflow to policyholder account balance from benefit payment.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 13J -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-13JReference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalanceBenefitPayment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of guaranteed benefit in excess of current account balance of policyholder account balance.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalanceNetAmountAtRisk**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of decrease in liability for policyholder account balance from policy charge.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 13J -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-13JReference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalancePolicyCharge**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash inflow to policyholder account balance from premium received.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 13J -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-13JReference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalancePremiumReceived**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_PolicyholderAccountBalanceRollForward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of decrease in liability for policyholder account balance from contract redemption and withdrawal of funds.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalanceSurrenderAndWithdrawal**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted-average crediting rate for liability for policyholder account balance, calculated at a point in time.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderAccountBalanceWeightedAverageCreditingRate**Namespace Prefix:**us-gaap_**Data Type:**dtr-types:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount due to policyholder. Includes, but is not limited to, unpaid policy dividend, retrospective refund, and undistributed earnings on participating business. Excludes future policy benefit and claim expense.
+ References
Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 29F -Publisher FASB -URI https://asc.fasb.org//1943274/2147480046/944-40-55-29FReference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7AReference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7AReference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(14)(a)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 7A -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-7A
+ Details**Name:**us-gaap_PolicyholderFunds**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=us-gaap_DeferredFixedAnnuityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_EquitySecuritiesByIndustryAxis=cinf_InvestmentcontractreservesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**srt_ProductOrServiceAxis=us-gaap_UniversalLifeMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |