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Retirement Benefits Activity (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2021
Subsidiaries [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Contribution Plan, Interest Credit 4.00%    
UA represented [Member] | employer contribution [Member] | Subsidiaries [Member] | Collective Bargaining Arrangement [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 4.00%    
IBEW represented | employer contribution [Member] | Subsidiaries [Member] | Collective Bargaining Arrangement [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Defined Contribution Plan, Employer Matching Contribution, Percent of Match 4.00%    
Supplemental Employee Retirement Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer contributions $ 1.5 $ 18.1  
Qualified Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Employer contributions $ 18.0 $ 18.0  
Forecast [Member] | Qualified Pension Benefits [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Aggregate expected contributions     $ 18.0
Forecast [Member] | Supplemental Employee Retirement Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Aggregate expected contributions     6.9
Forecast [Member] | Other Benefit [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Aggregate expected contributions     $ 0.3