Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Statement of Comprehensive Income [Abstract]
Net income	$ 636,187	$ 460,255	$ 100,141
Other comprehensive (loss) income, net of tax:
Net unrealized (loss) gain on derivatives, net of tax	(5,051)	(23,290)	2,165
Foreign currency translation adjustments	(5,937)	93,389	59,720
Pension adjustments, net of tax	601	(25,986)	(8,719)
Other comprehensive (loss) income, net of tax	(10,387)	44,113	53,166
Comprehensive income, net of tax	625,800	504,368	153,307
Noncontrolling interests in comprehensive income of consolidated subsidiaries, net of tax	(74,382)	(58,196)	(44,877)
Comprehensive income attributable to AECOM, net of tax	$ 551,418	$ 446,172	$ 108,430