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Pension and Other Postretirement Benefits (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Regulatory assets $ 1,930.5 $ 2,239.6  
Regulatory Liabilities $ 2,065.5 2,512.2  
Expected return on plan assets 5.70%    
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) $ 140.1 204.3  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) 0.7 0.2 $ (0.4)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 17.0 47.6 $ 14.8
Employer contributions 2.9 2.9  
Expected contribution 2.9    
Defined Benefit Plan, Accumulated Benefit Obligation 2,036.8 2,111.5  
Funded status of plan $ (59.3) $ 49.6  
Expected return on plan assets 5.70% 6.10% 7.00%
Settlement loss $ 10.5 $ 9.5 $ 18.5
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement 1.4    
Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 24.8 88.8  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (2.1) (3.2) (4.0)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) 11.9 10.0 $ 7.5
Employer contributions 21.6 23.0  
Expected contribution 21.8    
Funded status of plan $ 304.4 $ 315.1  
Expected return on plan assets 5.67% 5.83% 5.80%
Settlement loss $ 1.5 $ 0.0 $ 0.0
Significant Unobservable Inputs (Level 3) [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 0.0 0.0 $ 86.1
Percentage of investments 0.00%    
Unrecognized Pension Benefit And Other Postretirement Benefit Costs      
Defined Benefit Plan Disclosure [Line Items]      
Regulatory assets $ 583.3 739.1  
Regulatory Liabilities 0.0 0.1  
International Equities | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 230.1 205.0  
International Equities | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 48.2 40.7  
Commingled Funds | Short-Term Money Markets | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 78.8 14.8  
Commingled Funds | Short-Term Money Markets | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 7.6 7.7  
Commingled Funds | Short-Term Money Markets | Significant Unobservable Inputs (Level 3) [Member] | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Commingled Funds | Short-Term Money Markets | Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Commingled Funds | United States Equities | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 279.7 305.9  
Commingled Funds | United States Equities | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 14.0 12.1  
Commingled Funds | United States Equities | Significant Unobservable Inputs (Level 3) [Member] | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Commingled Funds | United States Equities | Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Commingled Funds | International Equities | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 176.8 148.1  
Commingled Funds | International Equities | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 24.1 20.1  
Commingled Funds | International Equities | Significant Unobservable Inputs (Level 3) [Member] | Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount 0.0 0.0  
Commingled Funds | International Equities | Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Plan Assets, Amount $ 0.0 $ 0.0