Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 17.5	$ 20.9	$ 34.9	$ 41.8
Expected return on plan assets	(18.1)	(17.1)	(36.1)	(34.3)
Amortization of prior service credits	0.0	0.0	0.0	0.0
Net periodic benefit (income) costs	(0.6)	3.8	(1.2)	7.5
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.2	0.3	0.5	0.6
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of prior service credits	(0.5)	(0.5)	(1.1)	(1.1)
Net periodic benefit (income) costs	$ (0.3)	$ (0.2)	$ (0.6)	$ (0.5)