Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Pension benefits
Net periodic benefit (income) costs
Interest cost	$ 81.8	$ 43.0	$ 41.2
Expected return on plan assets	(66.9)	(78.6)	(84.0)
Amortization of prior service credit	0.1	0.1	0.1
Actuarial (gain) loss	(35.0)	49.5	(75.1)
Net periodic benefit (income) costs	$ (20.0)	$ 14.0	$ (117.8)
Weighted-average plan assumptions
Discount rate for interest cost	5.45%	2.10%	1.91%
Expected long-term rate of return on plan assets	4.90%	4.10%	4.40%
Other postretirement benefits
Net periodic benefit (income) costs
Interest cost	$ 1.2	$ 0.7	$ 0.7
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service credit	(2.2)	(2.2)	(11.9)
Actuarial (gain) loss	(6.6)	(5.6)	0.8
Net periodic benefit (income) costs	$ (7.6)	$ (7.1)	$ (10.4)
Weighted-average plan assumptions
Discount rate for interest cost	5.41%	1.92%	1.76%
Expected long-term rate of return on plan assets	0.00%	0.00%	0.00%