Retirement Benefits - Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 3,838.0
Plan assets at end of year	4,192.2	$ 3,838.0
Net amount on balance sheet consists of:
Retirement benefits	(720.6)	(1,284.0)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	5,026.9	4,907.3
Service cost	90.1	91.1	$ 78.2
Interest cost	125.6	136.4	158.3
Actuarial (gains) losses	(162.6)	154.1
Acquisitions	0.0	(0.6)
Plan amendments	0.1	0.0
Plan participant contributions	2.8	3.1
Benefits paid	(156.9)	(285.0)
Settlements	(219.3)	(10.5)
Currency translation and other	45.1	31.0
Benefit obligation at end of year	4,751.8	5,026.9	4,907.3
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	3,838.0	3,753.1
Actual return on plan assets	653.7	266.0
Company contributions	34.9	84.1
Plan participant contributions	2.8	3.1
Benefits paid	(156.9)	(285.0)
Settlements	(219.3)	(10.5)
Currency translation and other	39.0	27.2
Plan assets at end of year	4,192.2	3,838.0	3,753.1
Funded status of plans	(559.6)	(1,188.9)
Net amount on balance sheet consists of:
Other assets	118.5	27.9
Compensation and benefits	(37.0)	(13.9)
Retirement benefits	(641.1)	(1,202.9)
Net amount on balance sheet	(559.6)	(1,188.9)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	57.0	60.7
Service cost	1.2	1.0	0.9
Interest cost	1.2	1.6	2.3
Actuarial (gains) losses	(4.9)	(1.4)
Acquisitions	0.0	0.0
Plan amendments	0.0	0.0
Plan participant contributions	3.1	3.0
Benefits paid	(8.8)	(7.8)
Settlements	0.0	0.0
Currency translation and other	2.7	(0.1)
Benefit obligation at end of year	51.5	57.0	60.7
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	5.7	4.8
Plan participant contributions	3.1	3.0
Benefits paid	(8.8)	(7.8)
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	(51.5)	(57.0)
Net amount on balance sheet consists of:
Other assets	0.0	0.0
Compensation and benefits	(5.7)	(5.8)
Retirement benefits	(45.8)	(51.2)
Net amount on balance sheet	$ (51.5)	$ (57.0)