Pension Benefits - Changes in Fair Value of Plan Assets and Changes in Accumulated Benefit Obligations (Details) - Pension Plans, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets at Beginning of Year	$ 579.8	$ 520.9
Actual Return on Plan Assets	(32.1)	86.4
Employer Contributions	5.1	0.0
Benefits Paid	(27.5)	(27.5)
Fair Value of Plan Assets at End of Year	525.3	579.8	$ 520.9
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligations at Beginning of Year	637.2	593.6
Service Cost	0.0	0.0	4.8
Interest Cost	20.3	20.6	20.1
Actuarial (Gains) Losses	49.5	(50.5)
Projected Benefit Obligation at End of Year	580.5	637.2	$ 593.6
Funded Status—Plan Assets in Deficit of Projected Benefit Obligation	(55.2)	(57.4)
Amount Recognized in Accumulated Other Comprehensive Income:
Unamortized Amount Reported in AOCI at End of Year	(144.4)	(137.2)
Accumulated Benefit Obligation at End of Year	580.3	637.0
Defined Benefit Plan, Plan Assets, Benefits Paid	$ 27.5	$ 27.5