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Pension and Other Postretirement Benefits (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         $ 2,084.1 $ 2,422.5  
Regulatory assets         2,237.5 1,801.2  
Regulatory Liabilities         $ 2,660.0 2,795.6  
Expected return on plan assets         7.00%    
Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         $ 1,867.7 2,160.0 $ 1,750.9
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)         (127.5) 76.1  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) $ (0.4) $ (0.7)     (0.4) (0.7) (0.2)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)         14.8 41.1 $ 48.5
Employer contributions     $ 277.0   2.9 282.3  
Expected contribution         3.0    
Defined Benefit Plan, Accumulated Benefit Obligation         1,965.6 2,170.4  
Funded status of plan         113.6 32.6  
Liability, Defined Benefit Plan [1]         $ 113.6 $ 32.6  
Expected return on plan assets         7.00% 7.25% 8.00%
Settlement loss 18.5 13.7     $ 18.5 $ 13.7 $ 0.0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Remeasurement due to Settlement         3.0    
Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         216.3 262.5 231.4
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)         5.0 (18.9)  
Defined Benefit Plan, Amortization of Prior Service Cost (Credit) (4.0) (4.4)     (4.0) (4.4) (4.9)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)         7.5 5.3 $ 8.0
Employer contributions         21.0 31.6  
Expected contribution         20.6    
Funded status of plan         276.2 293.8  
Liability, Defined Benefit Plan [1]         $ 276.2 $ 293.8  
Expected return on plan assets         5.80% 6.99% 7.85%
Settlement loss $ 0.0 $ 0.0     $ 0.0 $ 0.0 $ 0.0
Significant Unobservable Inputs (Level 3) [Member]              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         $ 86.1 98.9 $ 73.1
Percentage of investments         4.00%    
Significant Unobservable Inputs (Level 3) [Member] | Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount           0.0  
Scenario, Forecast | Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)       $ 45.5      
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)       (0.2)      
Amortization of transition obligation       0.0      
Scenario, Forecast | Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)       2.4      
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)       3.2      
Amortization of transition obligation       $ 0.0      
Unrecognized Pension Benefit And Other Postretirement Benefit Costs              
Defined Benefit Plan Disclosure [Line Items]              
Regulatory assets         $ 798.3 733.5  
Regulatory Liabilities         0.1 0.1  
International Equities | Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         165.5 351.0  
International Equities | Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         17.5 39.8  
Commingled Funds | Short-Term Money Markets | Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         18.3 [2] 34.1 [3]  
Commingled Funds | Short-Term Money Markets | Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         5.2 [2] 9.4 [3]  
Commingled Funds | United States Equities | Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         245.2 [2] 466.6 [3]  
Commingled Funds | United States Equities | Other Postretirement Benefits              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         10.4 [2] 12.2 [3]  
Commingled Funds | International Equities | Pension Plan              
Defined Benefit Plan Disclosure [Line Items]              
Defined Benefit Plan, Plan Assets, Amount         $ 122.3 [2] $ 265.1 [3]  
[1] recognize our Consolidated Balance Sheets underfunded and overfunded status of our various defined benefit postretirement plans, measured as the difference between the fair value of the plan assets and the benefit obligation.
[2] This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.
[3] This class of investments is measured at fair value using the net asset value per share and has not been classified in the fair value hierarchy.