Retirement Benefits (Schedule of Net Periodic Benefit Cost) (Details) - Defined Benefit Pension Plan - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018		Mar. 31, 2017
Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Service cost					$ 1.3
U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6.4	$ 6.9	$ 12.8	[1]	15.2	[1]
Interest cost	26.8	27.6	53.5		52.5
Expected return on plan assets	(50.4)	(51.7)	(100.8)		(104.4)
Prior service cost amortization	0.4	0.6	0.8		1.2
Actuarial loss amortization	22.2	20.9	43.9		47.0
Settlements	1.5	4.1	3.3		4.1
Curtailment	0.0	0.1	0.0		4.3
Special termination benefits	0.4	(0.1)	0.4		1.0
Other	0.0	0.0	0.0		0.0
Net periodic benefit cost	7.3	8.4	13.9		20.9
U.S. | Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	0.0	0.1	0.0		0.7
U.S. | Continuing Operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	7.3	8.3	13.9		20.2
International
Defined Benefit Plan Disclosure [Line Items]
Service cost	6.6	6.3	12.9	[1]	13.0	[1]
Interest cost	9.6	7.9	18.8		15.5
Expected return on plan assets	(21.1)	(18.3)	(41.3)		(36.8)
Prior service cost amortization	0.0	(0.1)	0.0		(0.1)
Actuarial loss amortization	10.3	13.2	20.3		27.1
Settlements	0.0	4.0	0.0		1.7
Curtailment	0.0	1.8	0.0		(1.3)
Special termination benefits	0.0	0.1	0.0		0.5
Other	0.1	(2.2)	0.6		0.5
Net periodic benefit cost	5.5	12.7	11.3		20.1
International | Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	0.0	3.4	0.0		4.1
International | Continuing Operations
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 5.5	$ 9.3	$ 11.3		$ 16.0

[1] Includes total service costs from discontinued operations of $1.3 for the six months ended 31 March 2017.