Employee Benefit Plans - Schedule of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2021
Pension Plan
Components of net periodic benefit cost
Service cost		$ 6.6	$ 10.3
Interest cost		16.6	17.0
Settlement			(135.4)
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 2.6	6.6	10.3	$ 11.5
Interest cost	$ 4.3	$ 16.6	$ 17.0	$ 16.4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Expected return on plan assets	$ (5.3)	$ (19.4)	$ (20.6)	$ (19.8)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of prior service cost	$ 0.6	$ 1.9	$ 2.2	$ 2.3
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Settlement			$ 33.6
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Settlement Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]			Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of net actuarial (gain) loss	$ 0.2	$ (2.4)	$ 1.0	$ 4.9
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Amortization of Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Expenses paid	$ 2.5	$ 1.0	$ 3.0	$ 3.0
Net periodic benefit cost	4.9	4.3	46.5	18.3
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial (gain) loss	(13.0)	(12.5)	(26.2)	(74.1)
Amortization of prior service cost	(0.6)	(1.9)	(2.2)	(2.3)
Settlement			(33.6)
Amortization of net actuarial gain (loss)	(0.2)	2.4	(1.0)	(4.9)
Total	$ (13.8)	$ (12.0)	$ (63.0)	$ (81.3)
Weighted-average assumptions
Discount rate	2.91%	5.09%	2.83%	2.71%
Expected return on plan assets	4.46%	6.50%	4.31%	4.89%
Postretirement Health and Other
Components of net periodic benefit cost
Service cost	$ 0.5	$ 1.7	$ 2.2	$ 2.2
Interest cost	$ 0.4	$ 2.1	$ 1.4	$ 1.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of prior service cost	$ (0.4)	$ (1.4)	$ (1.4)	$ (1.4)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Amortization of net actuarial (gain) loss	$ 0.1	$ (0.2)	$ 0.3	$ 0.3
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Amortization of Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax
Net periodic benefit cost	$ 0.6	$ 2.2	$ 2.5	$ 2.3
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial (gain) loss	2.2	7.5	(12.0)	(1.1)
Amortization of prior service cost	0.4	1.4	1.4	1.4
Amortization of net actuarial gain (loss)	(0.1)	0.2	(0.3)	$ (0.3)
Total	$ 2.5	$ 9.1	$ (10.9)
Weighted-average assumptions
Discount rate	2.61%	4.95%	2.62%	2.36%