Retirement Benefits - Schedule of Amounts Included in Accumulated Other Comprehensive Loss, Net of Tax (Details) - USD ($) $ in Millions	Sep. 30, 2025	Sep. 30, 2024
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	$ (154.0)	$ (153.0)
Net actuarial loss	475.0	571.0
Total	321.0	418.0
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service (credit) cost	4.0	4.0
Net actuarial loss	10.0	9.0
Total	$ 14.0	$ 13.0