Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5.1	$ 4.3	$ 10.2	$ 6.8
Interest cost	2.3	3.2	4.6	6.3
Expected return on assets	(3.2)	(3.6)	(6.6)	(5.9)
Settlement loss	0.0	0.0	10.2	0.0
Other	0.7	0.4	1.6	0.9
Total benefit cost (credit)	4.9	4.3	20.0	8.1
Retiree Health Care Plan
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.1	0.2	0.2	0.3
Prior service credit	(0.2)	(0.2)	(0.4)	(0.4)
Total benefit cost (credit)	$ (0.1)	$ 0.0	$ (0.2)	$ (0.1)