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Benefit Plans and Other Postretirement Benefits, Pension Benefits (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]        
Operating and maintenance expenses   $ 200.6 $ 201.9 $ 201.2
Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan        
Pension Benefits [Abstract]        
Total benefit obligation     1.0  
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Operating and maintenance expenses     2.0 2.0
Pension Benefits [Abstract]        
Total benefit obligation   143.4 139.8 156.8
Net benefit cost recognized for financial reporting   $ 6.4 $ 10.7 $ 10.5
Minimum number of years historical achieved weighted average annual returns are used to determine investment return assumptions (in years)   20 years    
Expected average long-term rate of return on assets (as a percent)   7.10% 7.10% 7.10%
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   100.00% 100.00%  
Pension Plan [Member] | Domestic and international equity securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   37.00% 37.00%  
Pension Plan [Member] | Long-duration fixed income and interest rate swap securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   30.00% 28.00%  
Pension Plan [Member] | Short-to-intermediate fixed income securities        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   14.00% 18.00%  
Pension Plan [Member] | Alternative investments        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   17.00% 15.00%  
Pension Plan [Member] | Cash        
Target Pension Asset Allocations [Abstract]        
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage   2.00% 2.00%  
Forecast [Member] | Pension Plan [Member]        
Pension Benefits [Abstract]        
Expected average long-term rate of return on assets for next fiscal year (as a percent) 7.10%      
Parent Company [Member] | Supplemental Executive Retirement Plan (SERP) and Nonqualified Pension Plan        
Pension Benefits [Abstract]        
Total benefit obligation     $ 39.0 $ 33.0
Net benefit cost recognized for financial reporting   $ 4.0 $ 4.0