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Income Taxes - Additional information (Details) - USD ($) $ in Millions | 12 Months Ended
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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) |  |

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