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Pension Benefits, Other Postretirement Benefits and Employee Savings and Investment Plans (Obligations and Funded Status) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Pension Benefits      
Change in plan assets:      
Fair value of plan assets as of January 1 $ 26.3 $ 33.5  
Actual return on plan assets 1.9 (5.6)  
Settlement (0.3) 0.0  
Employer contributions 0.3 0.0  
Benefit payments (1.5) (1.6)  
Fair value of plan assets as of December 31 26.7 26.3 $ 33.5
Change in plan benefit obligations:      
Fair value of plan benefit obligations as of January 1 21.3 28.7  
Service cost 0.0 0.0 0.0
Interest cost 1.1 0.7 0.7
Actuarial (gain) loss 0.8 (6.6)  
Settlement (0.3) 0.0  
Benefit payments (1.5) (1.5)  
Fair value of plan benefit obligations as of December 31 21.4 21.3 28.7
Amount overfunded (underfunded) 5.3 5.0  
Other Postretirement Benefits      
Change in plan assets:      
Fair value of plan assets as of January 1 0.0 0.0  
Actual return on plan assets 0.0 0.0  
Settlement 0.0 0.0  
Employer contributions 0.0 0.0  
Benefit payments 0.0 0.0  
Fair value of plan assets as of December 31 0.0 0.0 0.0
Change in plan benefit obligations:      
Fair value of plan benefit obligations as of January 1 1.4 1.4  
Service cost 0.1 0.1 0.1
Interest cost 0.1 0.0 0.0
Actuarial (gain) loss 0.0 (0.1)  
Settlement 0.0 0.0  
Benefit payments 0.0 0.0  
Fair value of plan benefit obligations as of December 31 1.6 1.4 $ 1.4
Amount overfunded (underfunded) $ (1.6) $ (1.4)