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Pension and Other Postretirement Benefit Plans - Defined Benefit Costs Recorded in Regulatory Long-Term Assets or Liabilities and Accumulated Other Comprehensive Income Expected to be Recognized over Next Fiscal Year (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Pension [Member]  
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]  
Net Loss $ 12.0
Prior Service Credit 0.0
Total Pension and Postretirement Health and Life Cost (Credit) 12.0
Postretirement Health and Life [Member]  
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]  
Net Loss 0.7
Prior Service Credit (1.8)
Total Pension and Postretirement Health and Life Cost (Credit) $ (1.1)