Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 17.4	$ 20.9
Expected return on plan assets	(18.0)	(17.2)
Amortization of prior service credits	0.0	0.0
Net periodic benefit (income) costs	(0.6)	3.7
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.3	0.3
Expected return on plan assets	0.0	0.0
Amortization of prior service credits	(0.6)	(0.6)
Net periodic benefit (income) costs	$ (0.3)	$ (0.3)