Retirement Plans & Other Postretirement Benefits - Reconciliation of Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of period	$ 350.4	$ 348.3
Service cost – benefits earned during the period	6.5	7.0	$ 7.9
Interest cost on projected benefit obligation	13.7	14.7	14.6
Plan participants' contributions	0.0	0.0
Plan amendments	1.4	0.0
Actuarial loss (gain)	25.4	8.7
Settlement loss	0.5	0.0
Benefit payments	(31.4)	(28.3)
Projected benefit obligation, end of period	366.5	350.4	348.3
Accumulated benefit obligation for all defined benefit pension plans	356.5	339.8
Other Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation, beginning of period	40.5	43.5
Service cost – benefits earned during the period	0.2	0.3	0.4
Interest cost on projected benefit obligation	1.5	1.7	2.0
Plan participants' contributions	1.2	1.1
Plan amendments	0.0	0.0
Actuarial loss (gain)	1.3	(1.6)
Settlement loss	0.0	0.0
Benefit payments	(4.7)	(4.5)
Projected benefit obligation, end of period	$ 40.0	$ 40.5	$ 43.5