Pension and Postretirement Benefit Costs - Schedule of Net Periodic Benefit (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Net periodic pension benefit	$ (8.7)	$ 2.7	$ (17.4)	$ 5.5
Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	0.1	0.1	0.1	0.1
Interest cost on projected benefit obligation	5.1	7.0	10.2	14.0
Expected return on plan assets	(5.7)	(7.4)	(11.4)	(14.8)
Net periodic pension benefit	(0.5)	(0.3)	(1.1)	(0.7)
Postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	0.3	1.1	0.5	2.2
Interest cost on projected benefit obligation	2.9	5.4	5.8	10.9
Expected return on plan assets	(0.2)	(0.4)	(0.4)	(0.8)
Amortization of prior service credit	(11.0)	(2.2)	(22.0)	(4.4)
Net periodic pension benefit	$ (8.0)	$ 3.9	$ (16.1)	$ 7.9