Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 10.6	$ 20.4
Interest cost	39.2	32.4
Expected return on plan assets	(51.3)	(59.4)
Amortization [Abstract]
Amortization of net actuarial (gain) loss	(1.0)	22.3
Net periodic postretirement benefit cost	(2.5)	15.7
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.1	0.2
Interest cost	0.6	0.3
Amortization [Abstract]
Amortization of net actuarial (gain) loss	0.1	0.2
Amortization of prior service credit	0.0	(0.2)
Net periodic postretirement benefit cost	$ 0.8	$ 0.5