Retirement Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5.1	$ 2.5
Interest cost	2.3	3.1
Expected return on assets	(3.4)	(2.3)
Settlement loss	10.2	0.0
Other	0.9	0.5
Total benefit cost (credit)	15.1	3.8
Retiree Health Care Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost	0.1	0.1
Expected return on assets	0.0	0.0
Settlement loss	0.0	0.0
Other	(0.2)	(0.2)
Total benefit cost (credit)	$ (0.1)	$ (0.1)