Postretirement Benefits Other Than Pensions - Components Of Comprehensive OPEB (Income) Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service Cost Earned During the Year	$ 0.2	$ 0.2	$ 0.2
Interest Cost on Accumulated Postretirement Benefit Obligation	0.3	0.4	0.4
Amortization of Prior Service Credit	(1.3)	(1.3)	(1.3)
Amortization of Accumulated Unrecognized OPEB Gain	(1.9)	(2.4)	(1.8)
Pension expense	(2.7)	(3.1)	(2.5)
Unrecognized OPEB (Gain) Loss Arising During the Year	1.9	(1.7)	(3.0)
Amortization of Prior Service Credit	1.3	1.3	1.3
Amortization of Accumulated Unrecognized OPEB Gain	1.9	2.4	1.8
Comprehensive OPEB (Income) Loss	$ 2.4	$ (1.1)	$ (2.4)