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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, 2019
Dec. 31, 2018
Other Postretirement Benefits Plan [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net (Gain) Loss $ (10.6) $ 4.7
Prior Service Credit Arising During the Period (34.6) (2.1)
Amortization of Prior Service Cost 2.8 2.1
Amortization of Loss (0.5) (0.8)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (42.9) 3.9
Pension [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net (Gain) Loss 20.4 5.8
Amortization of Prior Service Cost 0.1 0.1
Prior Service Credit Arising During the Period 0.0 (1.6)
Amortization of Loss (7.5) (11.4)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities $ 13.0 $ (7.1)