Retirement Benefits (Changes Recognized in Other Comprehensive Income on Pretax Basis) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
U.S.
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss arising during the period	$ 37.0	$ 11.9
Amortization of net actuarial gain (loss)	(48.8)	(58.4)
Prior service cost arising during the period	0.3	1.3
Amortization of prior service cost	(1.1)	(1.1)
Total	(12.6)	(46.3)
International
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss arising during the period	2.4	(56.5)
Amortization of net actuarial gain (loss)	(10.2)	(14.1)
Prior service cost arising during the period	0.0	0.0
Amortization of prior service cost	(1.0)	(0.9)
Total	$ (8.8)	$ (71.5)