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Pension and Postretirement Health and Life Benefits (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement $ (7.2) $ 4.0
Pension Plan, Defined Benefit [Member]    
Service costs 3.5 3.3
Interest costs 24.3 30.3
Expected return on plan assets (gains) (37.5) (36.4)
Amortization of prior service costs (credits), net (0.2) 0.0
Actuarial (gains) losses, net 8.9 (13.1)
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement 56.7 59.8
Net periodic benefit cost (credit) (1.0) (15.9)
Postretirement Health and Life Benefits Plans [Member]    
Service costs 0.0 0.0
Interest costs 4.8 6.0
Expected return on plan assets (gains) 0.0 0.0
Amortization of prior service costs (credits), net (0.1) (0.1)
Actuarial (gains) losses, net (1.7) 9.1
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement (1.6) 9.1
Net periodic benefit cost (credit) $ 3.0 $ 15.0