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Pension and Postretirement Health and Life Benefits (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Pension Plan, Defined Benefit [Member]        
Service costs $ 0.9 $ 0.8 $ 2.7 $ 2.4
Interest costs 6.0 7.5 18.2 22.7
Amortization of prior service costs (credits), net 0.0 0.0 (0.2) 0.0
Expected return on plan assets (gains) (9.3) (9.1) (28.1) (27.3)
Net periodic benefit cost (credit) (2.4) (0.8) (7.4) (2.2)
Postretirement Health and Life Benefits Plans [Member]        
Interest costs 1.2 1.5 3.6 4.5
Amortization of prior service costs (credits), net (0.1) 0.0 (0.1) (0.1)
Net periodic benefit cost (credit) $ 1.1 $ 1.5 $ 3.5 $ 4.4