Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 10.0	$ 10.0
Interest cost	34.0	36.0
Expected return on plan assets	(41.0)	(42.0)
Amortization of net actuarial loss	6.0	0.0
Net periodic postretirement benefit cost	9.0	4.0
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.0	0.0
Interest cost	0.0	1.0
Amortization of net actuarial loss	1.0	0.0
Net periodic postretirement benefit cost	$ 1.0	$ 1.0