|**Income Taxes**                       | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
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|**Income Tax Disclosure [Abstract]**   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Income Taxes                           | Income TaxesThe differences between the 21% statutory federal income tax rate and our effective income tax rate were as follows:(Dollars in millions)Three months ended September 30,Nine months ended September 30,2024202320242023Tax at statutory rate:$21921.0%$(31)21.0%$50021.0%$16521.0%Increase (decrease) resulting from:Tax-exempt income from municipal bonds(5)(0.5)(5)3.4(16)(0.7)(15)(1.9)Dividend received exclusion(6)(0.6)(5)3.4(16)(0.7)(16)(2.0)Other121.3(8)5.3241.1(8)(1.1)Provision (benefit) for income taxes$22021.2%$(49)33.1%$49220.7%$12616.0%The provision (benefit) for federal income taxes is based upon filing a consolidated income tax return for the company and its domestic subsidiaries.We continue to believe that after considering all positive and negative evidence of taxable income in the carryback and carryforward periods as permitted by law, it is more likely than not that all of the deferred tax assets on our U.S. domestic operations and those related to Cincinnati Global Underwriting Ltd.SM(Cincinnati Global) will be realized. As a result, we have no valuation allowance for our U.S. domestic operations or Cincinnati Global at both September 30, 2024, and December 31, 2023.During the third quarter of 2024, we were notified by the Internal Revenue Service (IRS) that the audit of tax years ended December 31, 2021 and 2020, has concluded. Despite this, the statute of limitations remains open through September of 2025.Cincinnati GlobalCincinnati Global had no operating loss carryforwards in the United States and $85 million and $100 million in the United Kingdom at September 30, 2024, and December 31, 2023, respectively. These Cincinnati Global losses can only be utilized within the Cincinnati Global group.
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|(Dollars in millions)                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|Tax at statutory rate:                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Increase (decrease) resulting from:    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Tax-exempt income from municipal bonds |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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|Provision (benefit) for income taxes   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
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