Joint Ventures and Variable Interest Entities - Schedule of Financial Information of Unconsolidated Joint Ventures Derived from Unaudited Financial Statements (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Joint Ventures and Variable Interest Entities [Line Items]
Current assets	$ 6,730,227	$ 7,055,759
TOTAL ASSETS	12,200,249	12,061,669
Current liabilities	5,928,816	6,374,081
TOTAL LIABILITIES	9,504,065	9,691,259
Joint ventures’ equity	2,696,184	2,370,410	$ 2,383,711	$ 2,605,379
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	12,200,249	12,061,669
AECOM’s investment in joint ventures	138,056	138,067
Joint ventures summarized financial information
Revenue	16,139,622	16,105,498	14,378,461
Cost of revenue	14,922,909	15,021,157	13,432,996
Gross profit	1,216,713	1,084,341	945,465
Net income	636,187	460,255	$ 100,141
Equity Method Investment, Nonconsolidated Investee or Group of Investees
Joint Ventures and Variable Interest Entities [Line Items]
Current assets	1,537,700	1,379,000
Non-current assets	708,000	799,900
TOTAL ASSETS	2,245,700	2,178,900
Current liabilities	1,107,800	976,300
Non-current liabilities	92,400	114,800
TOTAL LIABILITIES	1,200,200	1,091,100
Joint ventures’ equity	1,045,500	1,087,800
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	2,245,700	2,178,900
AECOM’s investment in joint ventures	138,100	138,100
Joint ventures summarized financial information
Revenue	2,887,600	2,145,800
Cost of revenue	2,822,500	1,972,100
Gross profit	65,100	173,700
Net income	$ 67,100	$ 168,600