Retirement Benefits - Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 3,788.3
Plan assets at end of year	3,754.8	$ 3,788.3
Net amount on balance sheet consists of:
Retirement benefits	(605.1)	(892.5)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	4,585.0	4,785.9
Service cost	88.9	97.0	$ 88.0
Interest cost	155.3	151.6	169.5
Actuarial gains	(257.1)	(221.9)
Plan amendments	(0.3)	(6.9)
Plan participant contributions	3.7	3.9
Benefits paid	277.7	251.9
Special termination benefit		0.5
Settlements	(10.4)	(13.8)
Curtailments	(3.5)	(1.0)
Currency translation and other	(24.4)	41.6
Benefit obligation at end of year	4,259.5	4,585.0	4,785.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	3,788.3	3,447.9
Actual return on plan assets	220.6	315.5
Company contributions	50.3	254.9
Plan participant contributions	3.7	3.9
Benefits paid	(277.7)	(251.9)
Settlements	(10.4)	(13.8)
Currency translation and other	(20.0)	31.8
Plan assets at end of year	3,754.8	3,788.3	3,447.9
Funded status of plans	(504.7)	(796.7)
Net amount on balance sheet consists of:
Other assets	30.6	10.6
Compensation and benefits	(12.3)	(11.7)
Retirement benefits	(523.0)	(795.6)
Net amount on balance sheet	(504.7)	(796.7)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	77.8	86.9
Service cost	1.3	1.4	1.3
Interest cost	2.4	2.5	3.3
Actuarial gains	(8.5)	(3.8)
Plan participant contributions	3.4	3.4
Benefits paid	13.5	13.4
Settlements	0.0	0.0
Curtailments	0.0	0.0
Currency translation and other	(0.5)	0.8
Benefit obligation at end of year	62.4	77.8	86.9
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	0.0	0.0
Actual return on plan assets	0.0	0.0
Company contributions	10.1	10.0
Plan participant contributions	3.4	3.4
Benefits paid	(13.5)	(13.4)
Settlements	0.0	0.0
Currency translation and other	0.0	0.0
Plan assets at end of year	0.0	0.0	$ 0.0
Funded status of plans	(62.4)	(77.8)
Net amount on balance sheet consists of:
Other assets	0.0	0.0
Compensation and benefits	(10.1)	(9.5)
Retirement benefits	(52.3)	(68.3)
Net amount on balance sheet	$ (62.4)	$ (77.8)