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, 2019 USD ($)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	$ 12.8
Prior Service Credit	(0.2)
Total Pension and Postretirement Health and Life Cost (Credit)	12.6
Postretirement Health and Life [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net Loss	1.0
Prior Service Credit	(8.0)
Total Pension and Postretirement Health and Life Cost (Credit)	$ (7.0)