Employee Benefit Plans - Components of Pension and Other Postretirement Benefit Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 20.6	$ 10.8	$ 61.8	$ 32.4
Expected return on plan assets	(16.8)	(19.7)	(50.4)	(59.1)
Amortization of prior service credits	0.1	0.1	0.1	0.1
Net periodic benefit costs (income)	3.9	(8.8)	11.5	(26.6)
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	0.4	0.1	1.2	0.5
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of prior service credits	(0.6)	(0.5)	(1.7)	(1.6)
Net periodic benefit costs (income)	$ (0.2)	$ (0.4)	$ (0.5)	$ (1.1)