Retirement Benefits - Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Pension Benefits
Components of net periodic benefit cost (income)
Service cost	$ 20.4	$ 22.7
Interest cost	32.4	31.3
Expected return on plan assets	(59.4)	(60.4)
Amortization:
Prior service cost	0.0	0.4
Net actuarial loss	22.3	36.7
Settlements	0.0	(0.2)
Net periodic benefit cost (credit)	15.7	30.5
Other Postretirement Benefits
Components of net periodic benefit cost (income)
Service cost	0.2	0.3
Interest cost	0.3	0.3
Amortization:
Prior service cost	(0.2)	(1.4)
Net actuarial loss	0.2	0.3
Net periodic benefit cost (credit)	$ 0.5	$ (0.5)