Joint Ventures and Variable Interest Entities - Schedule of Financial Information of Consolidated Joint Ventures (Details) - USD ($) $ in Thousands	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
Total AECOM deficit	2,492,584	2,184,205
Noncontrolling interests	203,600	186,205
TOTAL STOCKHOLDERS’ EQUITY	2,696,184	2,370,410	$ 2,383,711	$ 2,605,379
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	12,200,249	12,061,669
Consolidated Joint Ventures
Joint Ventures and Variable Interest Entities [Line Items]
Current assets	699,000	836,900
Non-current assets	84,400	83,100
TOTAL ASSETS	783,400	920,000
Current liabilities	591,400	763,600
Non-current liabilities	5,700	1,500
TOTAL LIABILITIES	597,100	765,100
Total AECOM deficit	(15,900)	(17,200)
Noncontrolling interests	202,200	172,100
TOTAL STOCKHOLDERS’ EQUITY	186,300	154,900
TOTAL LIABILITIES AND STOCKHOLDERS’ EQUITY	$ 783,400	$ 920,000