Joint Ventures and Variable Interest Entities - Schedule of Financial Information of Consolidated Joint Ventures (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Joint Ventures and Variable Interest Entities [Line Items]
Current assets	$ 6,999,358	$ 7,176,059
TOTAL ASSETS	11,818,827	12,061,669
Current liabilities	6,139,555	6,374,081
TOTAL LIABILITIES	9,419,284	9,691,259
Total AECOM deficit	2,204,010	2,184,205
Noncontrolling interests	195,533	186,205
TOTAL STOCKHOLDERS’ EQUITY	2,399,543	2,370,410	$ 2,428,218	$ 2,383,711
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	11,818,827	12,061,669
Consolidated Joint Ventures
Joint Ventures and Variable Interest Entities [Line Items]
Current assets	782,700	836,900
Non-current assets	83,100	83,100
TOTAL ASSETS	865,800	920,000
Current liabilities	699,800	763,600
Non-current liabilities	1,500	1,500
TOTAL LIABILITIES	701,300	765,100
Total AECOM deficit	(16,200)	(17,200)
Noncontrolling interests	180,700	172,100
TOTAL STOCKHOLDERS’ EQUITY	164,500	154,900
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 865,800	$ 920,000