| **Statutory Accounting Information (Tables)**   | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
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| **Staturory Accounting Information [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Statutory Net Income And Statutory Surplus      | The statutory net income and statutory capital and surplus are presented below:(Dollars in millions) Net income  Capital and surplus  Years ended December 31, At December 31,  2019 2018 2017 2019 2018The Cincinnati Insurance Company $558 $626 $401 $5,620 $4,919The Cincinnati Casualty Company 13 16 21 437 398The Cincinnati Indemnity Company 3 5 4 111 102The Cincinnati Specialty Underwriters Insurance Company 62 69 58 526 479The Cincinnati Life Insurance Company 19 — 12 204 191            |

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| - Definition
Staturory Accounting Information [Abstract].
+ References
No definition available.
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| - Definition
Tabular disclosure of how the entity's reporting under GAAP as of the balance sheet date differs from the results based on prescribed and permitted accounting practices of the state or country of domicile in which a relevant statutory filing is made, or differences in results based on the National Association of Insurance Commissioners (NAIC) prescribed practices, or a combination thereof. Describes the accounting practices used and the related monetary effect on statutory surplus, net income, and risk-based capital. If an insurance enterprise's risk-based capital would have triggered a regulatory event had it not used a permitted practice, that fact is disclosed in the financial statements. Permitted statutory accounting practices include practices not prescribed but allowed by the domiciliary state insurance department regulatory authority.
+ References
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