Postretirement Benefits Other Than Pensions - Components Of Comprehensive OPEB (Income) Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Unrecognized Postretirement Benefit Costs Arising During the Year	$ (14.2)	$ (3.0)	$ 106.4
Amortization of Accumulated Unrecognized Pension Loss	(5.3)	(23.1)	(7.9)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service Cost Earned During the Year	0.1	0.2	0.2
Interest Cost on Accumulated Postretirement Benefit Obligation	0.8	1.0	1.1
Amortization of Actuarial Loss	(1.4)	(1.4)	(1.8)
Pension Expense (Income) Recognized in Consolidated Statements of Income	(0.5)	(0.2)	(0.5)
Unrecognized Postretirement Benefit Costs Arising During the Year	(1.3)	0.4	(2.0)
Prior Service Credit Arising During the Year	(11.0)	0.0	0.0
Amortization of Accumulated Unrecognized Pension Loss	1.4	1.4	1.8
Comprehensive OPEB Expense	$ (11.4)	$ 1.6	$ (0.7)