Pension and Postretirement Benefit Costs - Schedule of Net Periodic Benefit (Benefit) Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined benefit pension plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 0.1	$ 0.5	$ 0.2	$ 1.5
Interest cost	7.0	8.4	21.0	25.1
Expected return on plan assets	(7.5)	(7.9)	(22.3)	(23.5)
Net periodic pension (benefit) cost	(0.4)	1.0	(1.1)	3.1
Postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	1.1	1.2	3.3	3.6
Interest cost	5.4	6.3	16.3	18.9
Expected return on plan assets	(0.3)	(0.2)	(1.1)	(0.4)
Amortization of prior service credit	(2.2)	(2.2)	(6.6)	(6.6)
Net actuarial loss	13.0	0.0	13.0
Net periodic pension (benefit) cost	$ 17.0	$ 5.1	$ 24.9	$ 15.5