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Pension and Postretirement Health and Life Benefits (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Pension Plan, Defined Benefit [Member]    
Service costs $ 0.8 $ 0.9
Interest costs 4.5 6.1
Amortization of prior service costs (credits), net (0.1) (0.1)
Expected return on plan assets (gains) (9.6) (9.4)
Net periodic benefit cost (credit) (4.4) (2.5)
Postretirement Health and Life Benefits Plans [Member]    
Interest costs 0.9 1.2
Net periodic benefit cost (credit) $ 0.9 $ 1.2