|**Statutory Accounting Information**                    | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|**Staturory Accounting Information [Abstract]**         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Statutory Accounting Information                        | Statutory Accounting InformationInsurance companies’ statutory financial statements are presented on the basis of accounting practices prescribed or permitted by applicable state insurance departments of domicile. Insurance companies use statutory accounting practices (SAP) as recognized by various states. We have adopted the National Association of Insurance Commissioners’ (NAIC)Accounting Practices and Proceduresmanual, version effective January 1, 2001, and updates through the current year as a component of prescribed or permitted practices by laws of the state of domicile. The primary differences between SAP and GAAP include the valuation of investment gains and losses, expensing of policy acquisition costs, actuarial assumptions for life insurance reserves and deferred income taxes based on differences in statutory and taxable income.Statutory net income and capital and surplus are determined in accordance with SAP prescribed or permitted by insurance regulatory authorities for five legal entities, our insurance subsidiary and its four insurance subsidiaries. Statutory capital and surplus for our insurance subsidiary, The Cincinnati Insurance Company, includes capital and surplus of its four insurance subsidiaries. All capital and surplus amounts exceed statutory risk-based capital requirements. The statutory net income and statutory capital and surplus are presented below:(Dollars in millions)Net incomeCapital and surplusYears ended December 31,At December 31,20182017201620182017The Cincinnati Insurance Company$626$401$434$4,919$5,094The Cincinnati Casualty Company162111398392The Cincinnati Indemnity Company544102100The Cincinnati Specialty Underwriters Insurance Company695857479436The Cincinnati Life Insurance Company—122191195
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|(Dollars in millions)                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|The Cincinnati Insurance Company                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|The Cincinnati Casualty Company                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|The Cincinnati Indemnity Company                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|The Cincinnati Specialty Underwriters Insurance Company |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|The Cincinnati Life Insurance Company                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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X- DefinitionDisclosure of Statutory Accounting Practices
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