Joint Ventures and Variable Interest Entities - Unconsolidated (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
Joint ventures' 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
AECOM's investment in unconsolidated joint ventures	341,452		341,452			328,906
Joint ventures summarized financial information
Revenue	3,213,658	3,265,573	6,480,374	6,578,728
Cost of revenue	3,003,706	3,070,314	6,070,218	6,199,099
Gross profit	209,952	195,259	410,156	379,629
Net income	48,513	$ 45,278	109,790	78,300
Unconsolidated Joint Ventures
Financial Information
Current assets	1,291,000		1,291,000			1,323,200
Non-current assets	1,018,000		1,018,000			1,001,600
TOTAL ASSETS	2,309,000		2,309,000			2,324,800
Current liabilities	782,000		782,000			845,800
Non-current liabilities	529,300		529,300			537,200
TOTAL LIABILITIES	1,311,300		1,311,300			1,383,000
Joint ventures' equity	997,700		997,700			941,800
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,309,000		2,309,000			2,324,800
AECOM's investment in unconsolidated joint ventures	$ 341,500		341,500			$ 328,900
Joint ventures summarized financial information
Revenue			925,100	1,229,400
Cost of revenue			888,100	1,197,100
Gross profit			37,000	32,300
Net income			$ 34,000	$ 30,300