Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Income Tax Contingency [Line Items]
Income (loss) from domestic operations	$ 392.7	$ 233.0	$ (129.2)
Income from foreign operations	522.9	485.2	342.6
Tax benefit related to deferred tax assets resulting from legal entity restructuring	20.1
Increase in tax benefit		38.4
Increase in tax expense related to uncertain tax position		23.0
Increase (decrease) in valuation allowance	(3.8)
Valuation allowance	157.1	160.9
Tax expense (benefit) related to uncertain tax positions	46.7	18.6	9.4
Net operating loss carryforwards	700.7	744.6
Gross deferred tax assets	609.4	678.9
Gross tax asset exclusive of tax liabilities realizable	452.3
Gross book-tax differences from non-U.S. subsidiaries	1,100.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	144.1	97.9
Unrecognized tax benefits	129.6	81.3	62.1
Unrecognized tax benefits that would be included in the effective tax rate if recognized	125.4
Accrued interest, excluding any related income tax benefits	23.8	27.7
Accrued penalties, excluding any related income tax benefits	3.2	3.5
Capital Loss Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward	51.7	181.2
Research Tax Credit Carryforward
Income Tax Contingency [Line Items]
Tax credit carryforward, valuation allowance	47.0
Research Tax Credit Carryforward | State
Income Tax Contingency [Line Items]
Tax credit carryforward	30.5
Research Tax Credit Carryforward | Foreign
Income Tax Contingency [Line Items]
Tax credit carryforward	0.2
Other Credits
Income Tax Contingency [Line Items]
Tax credit carryforward	3.7
ACAP Investments
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance		(21.0)
Reduction in deferred tax assets		20.2
Valuation allowance		9.3
AECOM Capital Impairment Charge
Income Tax Contingency [Line Items]
Valuation allowance			$ 21.0
Capital Losses
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance	(5.6)
Foreign Net Operating Losses And Currency Translation Adjustments
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance	$ 1.9
CANADA | Operating Losses and Other Deferred Tax Assets
Income Tax Contingency [Line Items]
Increase (decrease) in valuation allowance		(11.7)
HONG KONG
Income Tax Contingency [Line Items]
Tax expense (benefit) related to uncertain tax positions		$ (6.9)