Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 9.3	$ 10.6
Interest cost	36.6	39.2
Expected return on plan assets	(42.2)	(51.3)
Amortization [Abstract]
Amortization of net actuarial gain	(0.3)	(1.0)
Net periodic postretirement benefit cost	3.4	(2.5)
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.1	0.1
Interest cost	0.6	0.6
Amortization [Abstract]
Amortization of net actuarial gain	0.4	0.1
Net periodic postretirement benefit cost	$ 1.1	$ 0.8