Postretirement Benefits Other Than Pensions - Changes in Fair Value of Plan Assets and Changes in Accumulated Benefit Obligations (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets at Beginning of Year	$ 0.0	$ 0.0
Employer Contributions	0.9	3.4
Plan Participants' Contributions	0.5	1.0
Benefits Paid	(1.4)	(4.4)
Fair Value of Plan Assets at End of Year	0.0	0.0	$ 0.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligations at Beginning of Year	15.1	29.6
Service Cost	0.1	0.1	0.2
Interest Cost	0.4	0.8	1.0
Plan Participants' Contributions	0.5	1.0
Benefits Paid	(1.4)	(4.4)
Medicare Part D Subsidy Received	0.2	0.3
Plan Amendments	0.0	(11.0)
Actuarial Losses	(0.5)	(1.3)
Projected Benefit Obligation at End of Year	14.4	15.1	$ 29.6
Defined Benefit Plan, Funded Status of Plan	(14.4)	(15.1)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	$ 25.0	$ 27.6