Postretirement Benefits Other Than Pensions - Components Of Comprehensive OPEB (Income) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Unrecognized OPEB Gain Arising During the Year	$ 4.8	$ 8.0	$ (3.9)
Amortization of Accumulated Unrecognized OPEB Gain	3.0	(1.1)	0.6
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost Earned During the Year	0.2	0.2	0.1
Interest Cost on Accumulated Postretirement Benefit Obligation	0.4	0.4	0.4
Amortization of Prior Service Credit	(1.3)	(1.3)	0.0
Amortization of Accumulated Unrecognized OPEB Gain	(2.4)	(1.8)	(1.8)
Pension Income Recognized in Consolidated Statements of Income	(3.1)	(2.5)	(1.3)
Unrecognized OPEB Gain Arising During the Year	(1.7)	(3.0)	(0.5)
Prior Service Credit Arising During the Year from Plan Amendments	0.0	0.0	(1.3)	$ 11.0
Amortization of Prior Service Credit	1.3	1.3	0.0
Amortization of Accumulated Unrecognized OPEB Gain	2.4	1.8	1.8
Comprehensive OPEB Income	$ (1.1)	$ (2.4)	$ (1.3)