Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Amortization [Abstract]
Amortization of net actuarial loss (gain)	$ (29.0)
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 41.0	$ 37.0	$ 42.0
Interest cost	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 136.0	$ 147.0	$ 150.0
Expected return on plan assets	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Expected return on plan assets	$ (167.0)	$ (170.0)	$ (191.0)
Amortization [Abstract]
Amortization of net actuarial loss (gain)	$ 27.0	$ (1.0)	$ (2.0)
Settlement and curtailment charges	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Settlement and curtailment charges	$ 1.0	$ 0.0	$ 123.0
Net periodic benefit cost	38.0	13.0	122.0
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	$ 1.0	$ 1.0	$ 1.0
Interest cost	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 2.0	$ 3.0	$ 3.0
Expected return on plan assets	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Expected return on plan assets	$ 0.0	$ 0.0	$ 0.0
Amortization [Abstract]
Amortization of net actuarial loss (gain)	$ 2.0	$ 1.0	$ 0.0
Settlement and curtailment charges	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Settlement and curtailment charges	$ 0.0	$ 0.0	$ 0.0
Net periodic benefit cost	$ 5.0	$ 5.0	$ 4.0