Income Taxes - Deferred tax (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Deferred tax assets:
Compensation and benefit accruals not currently deductible	$ 92.2	$ 91.6
Net operating loss carryforwards	102.3	113.7
Self-insurance reserves	23.2	4.5
Research and experimentation and other tax credits	43.1	111.4
Pension liability	44.7	52.1
Accrued liabilities	295.1	280.4
Capital loss carryforward	64.0	64.4
Partnership investment	102.0
Other	7.1	5.3
Total deferred tax assets	773.7	723.4
Deferred tax liabilities:
Unearned revenue	(7.0)	(1.5)
Depreciation and amortization	(13.1)	(111.4)
Acquired intangible assets	(5.4)	(11.2)
Investment in subsidiaries	(10.7)	(10.8)
Right of use assets	(94.0)	(125.6)
Contingent consideration	(34.2)	(33.6)
Other	(15.5)
Total deferred tax liabilities	(179.9)	(294.1)
Valuation allowance	(171.2)	(154.4)
Net deferred tax assets	422.6	274.9
Income Taxes, additional disclosures
Capital loss carryforwards	199.4	205.2
Remaining gross tax asset exclusive of tax liabilities realized	602.5
Valuation allowance related to the AECOM Capital impairment charge	21.0
Increase in valuation allowance	16.8
Decrease in valuation allowance related to capital losses	3.3
Increase in valuation allowance related to state net operating losses and credits	0.9
Gross book-tax differences from non-U.S. subsidiaries	1,300.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	79.5	70.5
Unrecognized tax benefits	62.1	55.2	$ 46.4
Unrecognized tax benefits that would be included in the effective tax rate if recognized	60.7
Other Credits
Income Taxes, additional disclosures
Unused tax credits	10.4
Federal
Income Taxes, additional disclosures
Net operating loss carryforwards	757.5	848.0
Liability for unrecognized tax benefits, including potential interest and penalties, net of related tax benefit	62.1	55.2
State
Income Taxes, additional disclosures
Net operating loss carryforwards	757.5	$ 848.0
State \| Research and development
Income Taxes, additional disclosures
Unused tax credits	27.2
Foreign Tax Authority [Member]
Income Taxes, additional disclosures
Net operating loss carryforwards	757.5
Foreign Tax Authority [Member] \| Research and development
Income Taxes, additional disclosures
Unused tax credits	$ 9.6

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