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, 2018 USD ($)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	$ 7.3
Prior Service Credit	(0.2)
Total Pension and Postretirement Health and Life Cost (Credit)	7.1
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	0.4
Prior Service Credit	(1.7)
Total Pension and Postretirement Health and Life Cost (Credit)	$ (1.3)