Employee Benefit Plans - Schedule of Change in Benefit Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Change in benefit obligations
Benefit obligations at beginning of period	$ 1,600.0
Actuarial gain	(41.6)	$ 43.9	$ (74.3)
Benefit obligations at end of period	1,500.0	1,600.0
Amounts in the Consolidated Balance Sheets
Noncurrent benefit liabilities (c)	168.0	199.4
Pension benefits
Change in benefit obligations
Benefit obligations at beginning of period	1,585.2	2,132.9
Interest cost	81.8	43.0	41.2
Benefits paid	(130.4)	(128.0)
Actuarial gain	(52.7)	(458.3)
Transfers in	4.4	0.5
Settlements	(10.2)	(4.9)
Benefit obligations at end of period	1,478.1	1,585.2	2,132.9
Change in plan assets
Fair value of plan assets at beginning of period	1,438.1	1,987.0
Actual return on plan assets	41.3	(429.5)
Employer contributions	17.8	13.0
Benefits paid	(130.4)	(128.0)
Settlements	(10.2)	(4.9)
Transfers in	4.4	0.5
Fair value of plan assets at end of period	1,361.0	1,438.1	1,987.0
Unfunded status of the plans as of September 30	117.1	147.1
Assets for Plan Benefits, Defined Benefit Plan	38.6	33.7
Amounts in the Consolidated Balance Sheets
Current benefit liabilities (b)	7.7	9.1
Noncurrent benefit liabilities (c)	148.0	171.7
Net liabilities recognized	155.7	180.8
Balance in Accumulated other comprehensive loss
Prior service cost (credit)	$ 1.1	$ 1.2
Weighted-average plan assumptions
Discount rate	5.98%	5.58%
Pension benefits | Liability
Amounts in the Consolidated Balance Sheets
Net liabilities recognized	$ 117.1	$ 147.1
Other postretirement benefits
Change in benefit obligations
Benefit obligations at beginning of period	30.7	38.9
Interest cost	1.2	0.7	0.7
Benefits paid	(3.0)	(3.3)
Actuarial gain	(6.6)	(5.6)
Transfers in	0.0	0.0
Settlements	0.0	0.0
Benefit obligations at end of period	22.3	30.7	38.9
Change in plan assets
Fair value of plan assets at beginning of period	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	3.0	3.3
Benefits paid	(3.0)	(3.3)
Settlements	0.0	0.0
Transfers in	0.0	0.0
Fair value of plan assets at end of period	0.0	0.0	$ 0.0
Unfunded status of the plans as of September 30	22.3	30.7
Assets for Plan Benefits, Defined Benefit Plan	0.0	0.0
Amounts in the Consolidated Balance Sheets
Current benefit liabilities (b)	2.6	4.4
Noncurrent benefit liabilities (c)	19.7	26.3
Net liabilities recognized	22.3	30.7
Balance in Accumulated other comprehensive loss
Prior service cost (credit)	$ (16.7)	$ (18.9)
Weighted-average plan assumptions
Discount rate	5.98%	5.56%
Assumed pre-65 health care cost trend rate	5.50%	5.60%
Ultimate pre-65 health care cost trend rate	4.00%