Retirement Benefits (Changes Recognized in Other Comprehensive Income on Pretax Basis) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
U.S.
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain arising during the period	$ (167.7)	$ (189.8)
Amortization of net actuarial loss	(132.4)	(103.3)
Prior service cost arising during the period	0.1	1.9
Amortization of prior service cost	(1.6)	(2.3)
Total	(301.6)	(293.5)
International
Defined Benefit Plan Disclosure [Line Items]
Net actuarial gain arising during the period	(64.6)	(162.0)
Amortization of net actuarial loss	(43.7)	(55.7)
Prior service cost arising during the period	0.7	0.0
Amortization of prior service cost	0.0	0.1
Total	$ (107.6)	$ (217.6)