Retirement Benefits - Schedule of Net Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	$ 2,457.4
Plan assets at end of year	2,779.2	$ 2,457.4
Net amount on balance sheet consists of
Retirement benefits	(549.1)	(503.6)
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	2,750.8	3,165.6
Service cost	37.2	42.0	$ 70.9
Interest cost	146.5	149.7	135.6
Actuarial losses	321.8	81.1
Plan participant contributions	1.7	1.8
Benefits paid	(213.2)	(151.1)
Settlements	(4.8)	(585.6)
Currency translation and other	37.6	47.3
Benefit obligation at end of year	3,077.6	2,750.8	3,165.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at beginning of year	2,457.4	2,903.9
Actual return on plan assets	463.8	209.7
Company contributions	27.3	26.6
Plan participant contributions	1.7	1.8
Benefits paid	(213.2)	(151.1)
Settlements	(4.8)	(585.6)
Currency translation and other	47.0	52.1
Plan assets at end of year	2,779.2	2,457.4	2,903.9
Funded status of plans	(298.4)	(293.4)
Net amount on balance sheet consists of
Other assets	183.7	150.4
Compensation and benefits	(18.4)	(16.8)
Retirement benefits	(463.7)	(427.0)
Net amount on balance sheet	(298.4)	(293.4)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	46.4	44.2
Service cost	0.5	0.6	0.8
Interest cost	2.6	2.2	1.3
Actuarial losses	4.8	8.7
Plan participant contributions	4.2	4.2
Benefits paid	(13.0)	(13.3)
Settlements	0.0	0.0
Currency translation and other	(0.9)	(0.2)
Benefit obligation at end of year	44.6	46.4	44.2
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	8.8	9.1
Plan participant contributions	4.2	4.2
Benefits paid	(13.0)	(13.3)
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	(44.6)	(46.4)
Net amount on balance sheet consists of
Other assets	0.0	0.0
Compensation and benefits	(6.5)	(7.1)
Retirement benefits	(38.1)	(39.3)
Net amount on balance sheet	$ (44.6)	$ (46.4)