Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Amortization [Abstract]
Amortization of prior service cost (credit)	$ (0.1)
Amortization of net actuarial (gain) loss	(0.5)
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 37.2	$ 42.0	$ 70.9
Interest cost	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 146.5	$ 149.7	$ 135.6
Expected return on plan assets	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Expected return on plan assets	$ (169.5)	$ (190.6)	$ (230.7)
Amortization [Abstract]
Amortization of prior service cost (credit)	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of prior service cost (credit)	$ 0.0	$ 0.1	$ 0.6
Amortization of net actuarial (gain) loss	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of net actuarial (gain) loss	$ (1.1)	$ (2.6)	$ 59.4
Settlement and curtailment charges	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Settlement and curtailment charges	$ 0.1	$ 123.4	$ 38.6
Net periodic benefit cost	13.2	122.0	74.4
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	$ 0.5	$ 0.6	$ 0.8
Interest cost	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 2.6	$ 2.2	$ 1.3
Expected return on plan assets	$ 0.0	$ 0.0	$ 0.0
Amortization [Abstract]
Amortization of prior service cost (credit)	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of prior service cost (credit)	$ 0.0	$ 0.0	$ (0.8)
Amortization of net actuarial (gain) loss	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of net actuarial (gain) loss	$ 1.6	$ 0.5	$ 0.7
Settlement and curtailment charges	0.0	0.0	0.0
Net periodic benefit cost	$ 4.7	$ 3.3	$ 2.0