|**Debt (Tables)**                                                                                                           | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|--------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Debt Disclosure [Abstract]**                                                                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Schedule of Debt                                                                                                            | December 31 (dollars in millions)20212020Bank credit lines, average year-end interest rates of 5.0% for 2021 and —% for 2020$0.8$—Revolving credit agreement borrowings, average year-end interest rates of 1.1% for 2021 and —% for 202050.0—Term notes with insurance companies, expiring 2029-2034, average year-end interest rates of 3.1% for 2021 and 3.3% for 2020145.9113.2196.7113.2Long-term debt due within one year(6.8)(6.8)Long-term debt$189.9$106.4
|                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|December 31 (dollars in millions)                                                                                           | 2021                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Bank credit lines, average year-end interest rates of 5.0% for 2021 and —% for 2020                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Revolving credit agreement borrowings, average year-end interest rates of 1.1% for 2021 and —% for 2020                     | 50.0                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Term notes with insurance companies, expiring 2029-2034, average year-end interest rates of 3.1% for 2021 and 3.3% for 2020 | 145.9                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|                                                                                                                            | 196.7                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Long-term debt due within one year                                                                                          | (6.8)                                                                                                                                                                                                                                                                                                                                                                                                                                                              
|Long-term debt                                                                                                              | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Schedule of Maturities of Long Term Debt                                                                                    | Scheduled maturities of long-term debt within each of the five years subsequent to December 31, 2021 are as follows:Years ending December 31 (dollars in millions)Amount2022$6.8202310.0202410.0202510.0202679.8                                                                                                                                                                                                                                                   
|                                                                                                                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Years ending December 31 (dollars in millions)                                                                              | Amount                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|2022                                                                                                                        | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2023                                                                                                                        | 10.0                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|2024                                                                                                                        | 10.0                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|2025                                                                                                                        | 10.0                                                                                                                                                                                                                                                                                                                                                                                                                                                               
|2026                                                                                                                        | 79.8                                                                                                                                                                                                                                                                                                                                                                                                                                                               

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