Pension and Postretirement Benefits - Components of Net Periodic (Income) Cost Associated with Pension Plans and Postretirement Benefit Obligations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.3	$ 0.4
Interest cost	6.8	10.5
Expected return on plan assets	(20.8)	(21.9)
Amortization of Prior Service Cost (Credit)	0.4	0.0
Amortization of actuarial loss (gain)	0.2	0.0
Net periodic cost (income)	(12.1)	(11.0)
Postretirement Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0
Interest cost	0.0	0.0
Expected return on plan assets	0.0	0.0
Amortization of Prior Service Cost (Credit)	(0.1)	(0.1)
Amortization of actuarial loss (gain)	0.0	0.0
Net periodic cost (income)	$ (0.1)	$ (0.1)