Joint Ventures and Variable Interest Entities - Consolidated (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financial Information
Current assets	$ 5,697,176		$ 5,697,176			$ 6,173,543
TOTAL ASSETS	11,196,063		11,196,063			11,733,954
Current liabilities	5,283,478		5,283,478			5,521,715
TOTAL LIABILITIES	8,546,410		8,546,410			8,904,377
Total AECOM equity	2,531,442		2,531,442			2,712,470
Noncontrolling interests	118,211		118,211			117,107
TOTAL STOCKHOLDERS' EQUITY	2,649,653	$ 2,926,700	2,649,653	$ 2,926,700	$ 2,626,239	2,829,577	$ 3,020,177	$ 3,413,544
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	11,196,063		11,196,063			11,733,954
Revenue	3,213,658	$ 3,265,573	6,480,374	6,578,728
Consolidated Joint Ventures
Financial Information
Current assets	502,000		502,000			503,900
Non-current assets	74,500		74,500			74,800
TOTAL ASSETS	576,500		576,500			578,700
Current liabilities	434,300		434,300			400,300
Non-current liabilities	1,500		1,500			1,500
TOTAL LIABILITIES	435,800		435,800			401,800
Total AECOM equity	32,400		32,400			74,000
Noncontrolling interests	108,300		108,300			102,900
TOTAL STOCKHOLDERS' EQUITY	140,700		140,700			176,900
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 576,500		576,500			$ 578,700
Revenue			$ 627,300	$ 379,800