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, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets at Beginning of Year	$ 0	$ 0
Employer Contributions	3.4	2.9
Plan Participants' Contributions	1.2	1.2
Benefits Paid	(4.6)	(4.1)
Fair Value of Plan Assets at End of Year	0	0	0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligations at Beginning of Year	35.1	42.8
Service Cost	0.2	0.2	0.2
Interest Cost	1.1	1.2	1.6
Plan Participants' Contributions	1.2	1.2
Benefits Paid	(4.6)	(4.1)
Medicare Part D Subsidy Received	0.1	0
Actuarial Losses (Gains)	(2.0)	(6.2)
Projected Benefit Obligation at End of Year	31.1	35.1	42.8
Funded Status—Plan Assets in Excess (Deficit) of Projected Benefit Obligation	(31.1)	(35.1)
Unamortized Actuarial Gain Reported in AOCI at End of Year	$ (18.6)	$ (18.4)