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Pension and Postretirement Health and Life Benefits (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Defined Benefit Plan, Accumulated Benefit Obligation $ 757.9 $ 763.5
Expected defined benefit plan contributions for next fiscal year for qualified plans 11.0  
Expected defined benefit plan contributions for next fiscal year for non-qualified plans 0.4  
Expected defined benefit plan contributions for next fiscal year for postretirement health and life benefit plans 11.6  
Defined Contribution Plan, Cost 1.6 1.7
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement (7.2) 4.0
Pension plan assets transferred for annuitization 30.4  
Pension plan obligations transferred for annuitization $ 30.4  
Pension Plan, Defined Benefit [Member]    
Approximate Number of Plan Participants 4,600  
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement $ 56.7 $ 59.8
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.50% 3.30%
Other Postretirement Benefits Plan [Member]    
Approximate Number of Plan Participants 2,600  
Defined Benefit Plan, Actuarial (Gain) Loss From Remeasurement $ (1.6) $ 9.1
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.50% 3.30%