Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Net Periodic Benefit Cost
Non-service component cost	$ 0.4	$ 0.6	$ 0.9	$ 1.5	$ 1.0
Defined Benefit Pension Plans
Net Periodic Benefit Cost
Service cost	2.8	2.4	5.6	4.8
Interest cost	2.9	2.8	5.9	5.5
Expected return on assets	(2.7)	(2.6)	(5.5)	(5.2)
Other	0.3	0.7	0.7	1.3
Total benefit cost	3.3	3.3	6.7	6.4
Contributions to pension plans	2.3		4.2
Estimated employer contribution to pension plans during current fiscal year	12.3		12.3
Retiree Health Care Plan
Net Periodic Benefit Cost
Interest cost	0.1	0.2	0.2	0.3
Prior service credit	(0.2)	(0.2)	(0.4)	(0.3)
Total benefit cost	(0.1)	$ 0.0	(0.2)	$ 0.0
Contributions to retiree health care plan	$ 0.3		$ 0.6