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, 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	$ 507.5	$ 543.0
Actual Return on Plan Assets	29.9	(10.6)
Employer Contributions	9.0	0.0
Benefits Paid	(25.5)	(24.9)
Fair Value of Plan Assets at End of Year	520.9	507.5	$ 543.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligations at Beginning of Year	597.8	634.0
Service Cost	4.8	10.5	8.7
Interest Cost	20.1	25.7	24.8
Benefits Paid	(25.5)	(24.9)
Curtailment	(23.3)	0.0
Actuarial Losses (Gains)	(19.7)	47.5
Projected Benefit Obligation at End of Year	593.6	597.8	$ 634.0
Funded Status—Plan Assets in Deficit of Projected Benefit Obligation	(72.7)	(90.3)
Amount Recognized in Accumulated Other Comprehensive Income:
Accumulated Actuarial Loss	(144.7)	(152.2)
Prior Service Credit	0.0	0.4
Unamortized Amount Reported in AOCI at End of Year	(144.7)	(151.8)
Accumulated Benefit Obligation at End of Year	$ 593.5	$ 573.9