Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4.1	$ 2.6	$ 10.9	$ 8.2
Interest cost	3.2	2.9	9.5	8.8
Expected return on assets	(3.5)	(2.6)	(9.4)	(8.1)
Other	0.5	0.4	1.4	1.1
Total benefit cost (credit)	4.3	3.3	12.4	10.0
Retiree Health Care Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.1	0.1	0.4	0.3
Net loss	0.0	0.1	0.0	0.1
Prior service credit	(0.2)	(0.2)	(0.6)	(0.6)
Total benefit cost (credit)	$ (0.1)	$ 0.0	$ (0.2)	$ (0.2)