Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Net Periodic Benefit Cost
Net periodic benefit cost excluding service cost	$ 0.5	$ 0.9	$ 1.0
Defined Benefit Pension Plans
Net Periodic Benefit Cost
Service cost	2.8	2.4
Interest cost	3.0	2.7
Expected return on assets	(2.8)	(2.6)
Other	0.4	0.6
Total benefit cost	3.4	3.1
Contributions to pension plans	1.9
Estimated employer contribution to pension plans during current fiscal year	12.1
Retiree Health Care Plan
Net Periodic Benefit Cost
Service cost	0.0	0.0
Interest cost	0.1	0.1
Expected return on assets	0.0	0.0
Other	(0.2)	(0.1)
Total benefit cost	(0.1)	$ 0.0
Contributions to retiree health care plan	$ 0.3