Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Pension benefits
Net periodic benefit costs (income)
Interest cost	$ 83.4	$ 81.8	$ 43.0
Expected return on plan assets	(68.4)	(66.9)	(78.6)
Amortization of prior service credit	0.1	0.1	0.1
Actuarial (gain) loss	(5.1)	(35.0)	49.5
Net periodic benefit (income) costs	$ 10.0	$ (20.0)	$ 14.0
Weighted-average plan assumptions
Discount rate for interest cost	5.92%	5.45%	2.10%
Expected long-term rate of return on plan assets	5.30%	4.90%	4.10%
Other postretirement benefits
Net periodic benefit costs (income)
Interest cost	$ 1.2	$ 1.2	$ 0.7
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service credit	(2.2)	(2.2)	(2.2)
Actuarial (gain) loss	2.7	(6.6)	(5.6)
Net periodic benefit (income) costs	$ 1.7	$ (7.6)	$ (7.1)
Weighted-average plan assumptions
Discount rate for interest cost	5.92%	5.41%	1.92%
Expected long-term rate of return on plan assets	0.00%	0.00%	0.00%