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Pension and Other Postretirement Benefits (Schedule of Benefit Obligations in Excess of Fair Value) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Accumulated Benefit Obligation $ 2,036.8 $ 2,111.5  
Defined Benefit Plan, Benefit Obligation 2,058.4 2,130.5 $ 1,981.3
Defined Benefit Plan, Funded (Unfunded) Status of Plan 59.3 (49.6)  
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 140.1 204.3  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Other Change (121.3) 0.0  
Other Postretirement Benefits      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Benefit Obligation 590.8 576.5 $ 492.5
Defined Benefit Plan, Funded (Unfunded) Status of Plan (304.4) (315.1)  
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 24.8 88.8  
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Other Change 0.0 0.0  
Underfunded Plan [Member] | Pension Plan      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Benefit Plan, Accumulated Benefit Obligation 32.1 1,473.9  
Defined Benefit Plan, Benefit Obligation 32.1 1,492.9  
Defined Benefit Plan, Plan Assets, Amount 0.0 1,435.1  
Defined Benefit Plan, Funded (Unfunded) Status of Plan $ (32.1) $ (57.8)