Retirement Benefits - Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 3,753.1
Plan assets at end of year	3,838.0	$ 3,753.1
Net amount on balance sheet consists of:
Retirement benefits	(1,284.0)	(1,231.9)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	4,907.3	4,259.5
Service cost	91.1	78.2	$ 88.9
Interest cost	136.4	158.3	155.3
Actuarial losses (gains)	154.1	720.4
Acquisitions	(0.6)
Plan amendments		(4.9)
Plan participant contributions	3.1	3.4
Benefits paid	(285.0)	(263.1)
Settlements	(10.5)	(6.2)
Currency translation and other	31.0	(38.3)
Benefit obligation at end of year	5,026.9	4,907.3	4,259.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	3,753.1	3,754.8
Actual return on plan assets	266.0	263.6
Company contributions	84.1	30.9
Plan participant contributions	3.1	3.4
Benefits paid	(285.0)	(263.1)
Settlements	(10.5)	(6.2)
Currency translation and other	27.2	(30.3)
Plan assets at end of year	3,838.0	3,753.1	3,754.8
Funded status of plans	(1,188.9)	(1,154.2)
Net amount on balance sheet consists of:
Other assets	27.9	6.5
Compensation and benefits	(13.9)	(12.7)
Retirement benefits	(1,202.9)	(1,148.0)
Net amount on balance sheet	(1,188.9)	(1,154.2)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	60.7	62.4
Service cost	1.0	0.9	1.3
Interest cost	1.6	2.3	2.4
Actuarial losses (gains)	(1.4)	4.7
Plan participant contributions	3.0	3.4
Benefits paid	(7.8)	(12.8)
Settlements	0.0	0.0
Currency translation and other	(0.1)	(0.2)
Benefit obligation at end of year	57.0	60.7	62.4
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	4.8	9.4
Plan participant contributions	3.0	3.4
Benefits paid	(7.8)	(12.8)
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	(57.0)	(60.7)
Net amount on balance sheet consists of:
Other assets	0.0	0.0
Compensation and benefits	(5.8)	(7.3)
Retirement benefits	(51.2)	(53.4)
Net amount on balance sheet	$ (57.0)	$ (60.7)