| **Condensed Consolidated Statements of Comprehensive Income (Parenthetical) - USD ($) $ in Millions**                                                 | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Statement of Comprehensive Income [Abstract]**                                                                                                      |           |           |   |   |
| Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax                          | $ (109)        | $ (19)         | $ (393) | $ (33) |
| Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax | 0              | 1              | 0 | 2 |
| Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax                                            | $ 0            | $ 0            | $ 0 | $ 2 |

| X                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income, Change in Life Deferred Acquisition Costs, Life Policy Reserves and Other, Tax
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income Loss, Pension And Other Postretirement Benefit Plans, Net Actuarial Loss And Prior Service Cost Arising During Period, Tax
+ References
No definition available.
+ Details**Name:**cinf_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansNetActuarialLossAndPriorServiceCostArisingDuringPeriodTax**Namespace Prefix:**cinf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| - Definition
Amount, before adjustment, of tax expense (benefit) for unrealized holding gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Excludes tax expense (benefit) for unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI https://asc.fasb.org/extlink&oid=126968391&loc=d3e640-108580Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (f) -URI https://asc.fasb.org/extlink&oid=126968391&loc=SL7669646-108580
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| X                                                                                                                                                                                                                         |
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| - References
No definition available.
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