Pension and Postretirement Plans - Schedule of Pension and Postretirement Costs (Benefits) (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended	8 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 07, 2021	Sep. 30, 2020	Sep. 07, 2021	Sep. 30, 2020
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	0.9	2.7	4.6	10.2	14.2
Expected return on plan assets	(2.1)	(5.2)	(7.1)	(19.9)	(21.6)
Amortization of:
Prior service benefit	0.0	0.0	0.0	0.0	0.0
Actuarial loss	0.0	4.5	4.5	16.0	13.5
Pension settlement charges	0.0			0.0	1.1
Pension / postretirement costs	(1.2)	2.0	2.0	6.3	7.2
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	0.0	0.1	0.1	0.3	0.3
Other components of pension and postretirement benefit plans expense:
Interest cost on projected benefit obligation	0.1	0.5	0.9	1.8	2.8
Expected return on plan assets	0.0	0.0	0.0	0.0	0.0
Amortization of:
Prior service benefit	0.0	(0.4)	(0.6)	(1.7)	(1.9)
Actuarial loss	0.0	0.0	0.3	0.0	0.9
Pension settlement charges	0.0			0.0	0.0
Pension / postretirement costs	$ 0.1	$ 0.2	$ 0.7	$ 0.4	$ 2.1