Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Amortization:
Prior service cost (credit)	$ 35.8
Net actuarial loss	142.5
Pension Benefits
Components of net periodic benefit cost
Service cost	90.1	$ 91.1	$ 78.2
Interest cost	125.6	136.4	158.3
Expected return on plan assets	(241.3)	(244.8)	(244.7)
Amortization:
Prior service cost (credit)	1.4	0.9	1.2
Net actuarial loss	141.4	147.3	77.8
Settlements	39.8	0.0	1.2
Net periodic benefit cost (income)	157.0	130.9	72.0
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	1.2	1.0	0.9
Interest cost	1.2	1.6	2.3
Expected return on plan assets	0.0	0.0	0.0
Amortization:
Prior service cost (credit)	(5.4)	(5.4)	(5.4)
Net actuarial loss	1.1	1.4	0.9
Settlements	0.0	0.0	0.0
Net periodic benefit cost (income)	$ (1.9)	$ (1.4)	$ (1.3)