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, 2016	Dec. 31, 2015	Dec. 31, 2014
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	3.4	3.4
Plan Participants' Contributions	1.0	1.2
Benefits Paid	(4.4)	(4.6)
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	29.6	31.1
Service Cost	0.1	0.2	0.2
Interest Cost	0.8	1.0	1.1
Plan Participants' Contributions	1.0	1.2
Benefits Paid	(4.4)	(4.6)
Medicare Part D Subsidy Received	0.3	0.3
Plan Amendments	(11.0)	0.0
Actuarial Losses (Gains)	(1.3)	0.4
Projected Benefit Obligation at End of Year	15.1	29.6	$ 31.1
Defined Benefit Plan, Funded Status of Plan	(15.1)	(29.6)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Gains (Losses), before Tax	$ (27.6)	$ (16.8)