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Retirement Benefits Plans (Postretirement Benefit Plans (Benefit) Expense Components) (Details) - Postretirement benefit plans - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Interest cost on benefit obligations $ 0.7 $ 1.1 $ 1.0
Amortization of:      
Actuarial losses 0.7 0.6 0.6
Prior service benefit (0.5) (0.6) (0.7)
Total expense $ 0.9 $ 1.1 $ 0.9