Amount of decrease in the deferred tax asset due to sale of investments.

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Amount of increase in income tax expense related to uncertain tax positions partially offset to the income tax benefit.

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Amount of increase in income tax benefit related to state income taxes due to apportionment factor.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset.

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Income Taxes - Additional information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Income taxes
Increase in tax benefit	$ 38.4
Increase in tax expense related to uncertain tax position	23.0
Increase (decrease) in valuation allowance	(10.3)
Tax expense (benefit) related to uncertain tax positions	18.6	$ 9.4	$ 15.4
Valuation allowance	160.9	171.2
ACAP Investments
Income taxes
Increase (decrease) in valuation allowance	(21.0)
Reduction in deferred tax assets	20.2
Valuation allowance	9.3
AECOM Capital impairment charge
Income taxes
Valuation allowance		$ 21.0
CANADA \| Net operating losses and other deferred tax assets
Income taxes
Increase (decrease) in valuation allowance	(11.7)
HONG KONG
Income taxes
Tax expense (benefit) related to uncertain tax positions	(6.9)
Foreign \| Net operating losses and other deferred tax assets
Income taxes
Increase (decrease) in valuation allowance			$ (21.9)
UNITED STATES \| ACAP Investments
Income taxes
Increase (decrease) in valuation allowance	$ (11.7)