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Benefit Plans and Other Postretirement Benefits, Pension Plan Benefit Obligations, Cash Flows and Benefit Costs (Details)
$ in Millions
1 Months Ended 12 Months Ended
Jan. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Plan
Dec. 31, 2018
USD ($)
Plan
Dec. 31, 2017
USD ($)
Plan
Components of Net Periodic Benefit Cost (Credit) [Abstract]          
Settlement Charge Recognized in Operating and Maintenance Expenses     $ 200.6 $ 201.9 $ 201.2
Significant Assumptions Used to Measure Costs [Abstract]          
Liability, Defined Benefit Plan, Noncurrent     (33.4) (44.5)  
Pension Plan [Member]          
Defined Benefit Plan Disclosure [Line Items]          
Defined Benefit Plan, Benefit Obligation, Contributions by Plan Participant     0.0 0.0  
Accumulated Benefit Obligation at Dec. 31     131.9 129.4  
Change in Projected Benefit Obligation [Roll Forward]          
Obligation at Jan. 1 $ 143.4 $ 143.4 139.8 156.8  
Service cost     4.4 4.8 4.6
Interest cost     5.7 5.4 6.2
Plan amendments     0.1 0.0  
Actuarial loss     7.5 (13.4)  
Benefit payments     (14.1) (13.8)  
Obligation at Dec. 31     143.4 139.8 156.8
Change in Fair Value of Plan Assets [Roll Forward]          
Fair value of plan assets at Jan. 1 124.1 $ 124.1 109.5 124.9  
Actual return (loss) on plan assets     21.4 (11.1)  
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant     0.0 0.0  
Employer contributions     7.3 9.5  
Benefit payments     (14.1) (13.8)  
Fair value of plan assets at Dec. 31     124.1 109.5 124.9
Defined Benefit Plan, Plan Assets, Payment for Settlement       10.4  
Funded Status of Plans at Dec. 31 [Abstract]          
Funded status     (19.3) (30.3)  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost [Abstract]          
Net loss     64.3 74.3  
Prior service (credit) cost     (0.2) (0.3)  
Total     64.1 74.0  
Amounts Not Yet Recognized as Components of Net Periodic Benefit Cost Have Been Recorded as Follows Based Upon Expected Recovery in Rates [Abstract]          
Current regulatory assets     4.3 5.3  
Noncurrent regulatory assets     59.8 68.7  
Total     $ 64.1 $ 74.0  
Significant Assumptions Used to Measure Benefit Obligations [Abstract]          
Discount rate for year-end valuation (as a percent)     3.49% 4.31%  
Expected average long-term increase in compensation level (as a percent)     3.75% 3.75%  
Cash Flows [Abstract]          
Payment for Pension Benefits     $ 7.0 $ 10.0 9.0
Components of Net Periodic Benefit Cost (Credit) [Abstract]          
Service cost     4.4 4.8 4.6
Interest cost     5.7 5.4 6.2
Expected return on plan assets     (8.3) (9.0) (9.2)
Amortization of prior service cost (credit)     0.0 0.0 0.1
Amortization of net loss     4.4 5.7 5.9
Settlement charge     0.0 7.2 7.1
Net periodic benefit cost     6.2 14.1 14.7
Net benefit cost recognized for financial reporting     $ 6.4 10.7 10.5
Settlement Charge Recognized in Operating and Maintenance Expenses       $ 2.0 $ 2.0
Significant Assumptions Used to Measure Costs [Abstract]          
Discount rate (as a percent)     4.31% 3.63% 4.13%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase     3.75% 3.75% 3.75%
Liability, Defined Benefit Plan, Current     $ 0.0 $ 0.0  
Liability, Defined Benefit Plan, Noncurrent     (19.3) (30.3)  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position     (19.3) (30.3)  
Defined Benefit Plan Credits (Costs) Not Recognized Due To Effects of Regulation     $ 0.2 $ (3.4) $ (4.2)
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets     7.10% 7.10% 7.10%
Subsequent Event          
Significant Assumptions Used to Measure Costs [Abstract]          
Expected average long-term rate of return on assets for next fiscal year (as a percent)   7.10%      
Subsequent Event | Pension Plan [Member]          
Cash Flows [Abstract]          
Payment for Pension Benefits 7.0        
Parent Company [Member] | Pension Plan [Member]          
Change in Fair Value of Plan Assets [Roll Forward]          
Defined Benefit Plan, Plan Assets, Payment for Settlement       $ 198.0  
Cash Flows [Abstract]          
Number of pension plans to which contributions were made | Plan     4 4 4
Payment for Pension Benefits     $ 154.0 $ 150.0 $ 162.0
Parent Company [Member] | Subsequent Event | Pension Plan [Member]          
Cash Flows [Abstract]          
Payment for Pension Benefits $ 150.0