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Pension and Other Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Pension    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net Loss $ 2.5 $ 11.4
Amortization of Prior Service Credit 0.1 0.1
Prior Service Credit Arising During the Period 0.0 0.8
Amortization of Gain (Loss) (5.7) (11.4)
Amortization of Prior Service Credit (0.1) (0.1)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (3.1) 0.9
Postretirement Health and Life Obligation and Funded Status    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net Loss (18.3) 3.9
Prior Service Credit Arising During the Period (29.1) 0.0
Amortization of Prior Service Credit 8.4 7.5
Amortization of Gain (Loss) 2.7 (0.4)
Amortization of Prior Service Credit (8.4) (7.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities $ (36.3) $ 11.0