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, 2020	Dec. 31, 2019
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	$ 8.1	$ (10.6)
Prior Service Credit Arising During the Period	0.0	(34.6)
Amortization of Prior Service Cost	8.0	2.8
Amortization of Loss	(1.0)	(0.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	15.1	(42.9)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net (Gain) Loss	68.4	20.4
Amortization of Prior Service Cost	0.2	0.1
Amortization of Loss	(12.8)	(7.5)
Total Recognized in Other Comprehensive Income and Regulatory Assets or Liabilities	$ 55.8	$ 13.0