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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, 2017
Dec. 31, 2016
Pension [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net (Gain) Loss $ (4.3) $ 7.2
Amortization of Loss (9.9) (9.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities (14.2) (2.3)
Postretirement Health and Life [Member]    
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]    
Net (Gain) Loss 1.6 13.5
Prior Service Credit Arising During the Period (1.8) 0.0
Amortization of Prior Service Credit 2.0 2.9
Amortization of Loss (0.3) (0.2)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities $ 1.5 $ 16.2