Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Pension benefits
Net periodic benefit costs (income)
Interest cost	$ 43.0	$ 41.2	$ 59.6
Expected return on plan assets	(78.6)	(84.0)	(85.3)
Amortization of prior service credit	0.1	0.1	0.1
Actuarial loss (gain)	49.5	(75.1)	(21.6)
Net periodic benefit (income) costs	$ 14.0	$ (117.8)	$ (47.2)
Weighted-average plan assumptions
Discount rate for interest cost	2.10%	1.91%	2.80%
Expected long-term rate of return on plan assets	4.10%	4.40%	4.70%
Other postretirement benefits
Net periodic benefit costs (income)
Interest cost	$ 0.7	$ 0.7	$ 1.1
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service credit	(2.2)	(11.9)	(11.8)
Actuarial loss (gain)	(5.6)	0.8	3.0
Net periodic benefit (income) costs	$ (7.1)	$ (10.4)	$ (7.7)
Weighted-average plan assumptions
Discount rate for interest cost	1.92%	1.76%	2.68%
Expected long-term rate of return on plan assets	0.00%	0.00%	0.00%