Pension and Postretirement Benefits - Components of Net Periodic Cost (Income) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 07, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net periodic cost (income)	$ 1.2	$ 10.6	$ 9.4	$ 11.0
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.3	1.7	1.4	3.7
Interest cost	6.8	42.2	46.7	57.2
Expected return on plan assets	(10.6)	(87.7)	(82.7)	(96.4)
Amortization of Prior Service Cost (Credit)	0.0	0.0	0.0	0.2
Recognized actuarial loss (gain)	4.0	0.0	0.0	42.5
Net periodic cost (income)	0.5	(43.8)	(34.6)	7.2
Postretirement Benefit Obligations
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.0	0.0	0.5
Interest cost	0.0	0.0	0.1	0.3
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of Prior Service Cost (Credit)	(0.1)	(0.4)	0.0	(0.3)
Recognized actuarial loss (gain)	(0.1)	0.0	0.0	(1.5)
Net periodic cost (income)	$ (0.2)	$ (0.4)	$ 0.1	$ (1.0)