Retirement Benefits - Schedule of Components of Net Periodic Benefit Cost (Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Amortization [Abstract]
Amortization of prior service cost (credit)	$ 123.5
Amortization of net actuarial (gain) loss	(2.1)
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 42.0	$ 70.9	$ 90.1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 149.7	$ 135.6	$ 125.6
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Expected Return (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Expected return on plan assets	$ (190.6)	$ (230.7)	$ (241.3)
Amortization [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of prior service cost (credit)	$ 0.1	$ 0.6	$ 1.4
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of net actuarial (gain) loss	$ (2.6)	$ 59.4	$ 141.4
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Settlement and Curtailment Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Settlement and curtailment charges	$ 123.4	$ 38.6	$ 39.8
Net periodic benefit cost (income)	122.0	74.4	157.0
Other Postretirement Benefits
Components of net periodic benefit cost
Service cost	$ 0.6	$ 0.8	$ 1.2
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Interest Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Interest cost	$ 2.2	$ 1.3	$ 1.2
Expected return on plan assets	$ 0.0	$ 0.0	$ 0.0
Amortization [Abstract]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Amortization of Prior Service Cost (Credit), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of prior service cost (credit)	$ 0.0	$ (0.8)	$ (5.4)
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Immediate Recognition of Actuarial Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Other Income (Expense)	Other Income (Expense)	Other Income (Expense)
Amortization of net actuarial (gain) loss	$ 0.5	$ 0.7	$ 1.1
Settlement and curtailment charges	0.0	0.0	0.0
Net periodic benefit cost (income)	$ 3.3	$ 2.0	$ (1.9)