Pension and Postretirement Plans - Schedule of Pension and Postretirement Costs Benefit (Details) - USD ($) $ in Millions	4 Months Ended	8 Months Ended	12 Months Ended
	Dec. 31, 2021	Sep. 07, 2021	Dec. 31, 2023	Dec. 31, 2022
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.0	$ 0.0	$ 0.0	$ 0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals
Interest cost on projected benefit obligation	$ 4.7	$ 10.2	$ 22.0	$ 16.3
Expected return on plan assets	(10.2)	(19.9)	(20.8)	(27.5)
Amortization of:
Prior service benefit	0.0	0.0	0.0	0.0
Actuarial (gain) loss	0.0	16.0	(0.3)	0.0
Pension/postretirement cost (benefit)	(5.5)	6.3	0.9	(11.2)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 0.1	$ 0.3	$ 0.5	$ 0.8
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals	Other Components Of Pension And Postretirement Benefit Plans Expense Reversals
Interest cost on projected benefit obligation	$ 0.8	$ 1.8	$ 5.2	$ 4.2
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of:
Prior service benefit	0.0	(1.7)	(0.7)	(0.2)
Actuarial (gain) loss	0.0	0.0	(4.7)	(0.6)
Pension/postretirement cost (benefit)	$ 0.9	$ 0.4	$ 0.3	$ 4.2