Retirement Benefits - Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 2,903.9
Plan assets at end of year	2,457.4	$ 2,903.9
Net amount on balance sheet consists of
Retirement benefits	(503.6)	(471.2)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	3,165.6	4,751.8
Service cost	42.0	70.9	$ 90.1
Interest cost	149.7	135.6	125.6
Actuarial losses (gains)	81.1	(1,216.8)
Plan amendments	0.0	4.6
Plan participant contributions	1.8	2.2
Benefits paid	(151.1)	(153.8)
Settlements and curtailments	(585.6)	(320.4)
Currency translation and other	47.3	(108.5)
Benefit obligation at end of year	2,750.8	3,165.6	4,751.8
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	2,903.9	4,192.2
Actual return on plan assets	209.7	(768.0)
Company contributions	26.6	54.3
Plan participant contributions	1.8	2.2
Benefits paid	(151.1)	(153.8)
Settlements and curtailments	(585.6)	(312.2)
Currency translation and other	52.1	(110.8)
Plan assets at end of year	2,457.4	2,903.9	4,192.2
Funded status of plans	(293.4)	(261.7)
Net amount on balance sheet consists of
Other assets	150.4	158.8
Compensation and benefits	(16.8)	(15.1)
Retirement benefits	(427.0)	(405.4)
Net amount on balance sheet	(293.4)	(261.7)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	44.2	51.5
Service cost	0.6	0.8	1.2
Interest cost	2.2	1.3	1.2
Actuarial losses (gains)	8.7	(1.1)
Plan amendments	0.0	0.0
Plan participant contributions	4.2	3.8
Benefits paid	(13.3)	(11.7)
Settlements and curtailments	0.0	0.0
Currency translation and other	(0.2)	(0.4)
Benefit obligation at end of year	46.4	44.2	51.5
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	9.1	7.9
Plan participant contributions	4.2	3.8
Benefits paid	(13.3)	(11.7)
Settlements and curtailments	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	(46.4)	(44.2)
Net amount on balance sheet consists of
Other assets	0.0	0.0
Compensation and benefits	(7.1)	(6.4)
Retirement benefits	(39.3)	(37.8)
Net amount on balance sheet	$ (46.4)	$ (44.2)