Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 9.3	$ 9.4	$ 27.8	$ 30.6
Interest cost	36.6	35.3	109.9	114.0
Expected return on plan assets	(42.2)	(44.0)	(126.9)	(147.0)
Amortization of prior service credit	0.0	0.1	0.0	0.1
Amortization of net actuarial (gain) loss	(0.3)	0.2	(0.8)	(1.8)
Settlement charge	0.0	2.2	0.0	120.1
Net periodic postretirement benefit cost	3.4	3.2	10.0	116.0
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.1	0.2	0.4	0.4
Interest cost	0.6	0.6	1.9	1.7
Amortization of net actuarial (gain) loss	0.4	0.1	1.2	0.4
Net periodic postretirement benefit cost	$ 1.1	$ 0.9	$ 3.5	$ 2.5