Retirement Benefits (Changes Recognized in Other Comprehensive Income on Pretax Basis) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
U.S.
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) arising during the period		$ 262.9	$ (167.7)
Amortization of net actuarial loss		(71.5)	(132.4)
Prior service cost arising during the period		1.1	0.1
Amortization of prior service cost		(1.1)	(1.6)
Total		191.4	(301.6)
International
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain) arising during the period		161.5	(64.6)
Amortization of net actuarial loss		(11.1)	(43.7)
Prior service cost arising during the period	$ 4.7	4.7	0.7
Amortization of prior service cost		0.0	0.0
Total		$ 155.1	$ (107.6)