Joint Ventures and Variable Interest Entities - Unconsolidated (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Financial Information
Current assets	$ 5,930,268		$ 6,173,543
TOTAL ASSETS	11,443,018		11,733,954
Current liabilities	5,513,463		5,521,715
TOTAL LIABILITIES	8,816,779		8,904,377
Joint ventures' equity	2,626,239	$ 3,020,177	2,829,577	$ 3,413,544
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	11,443,018		11,733,954
AECOM's investment in unconsolidated joint ventures	327,500		328,906
Joint ventures summarized financial information
Revenue	3,266,716	3,313,155
Cost of revenue	3,066,512	3,128,785
Gross profit	200,204	184,370
Net income	61,277	33,022
Unconsolidated Joint Ventures
Financial Information
Current assets	1,310,500		1,323,200
Non-current assets	1,102,000		1,001,600
TOTAL ASSETS	2,412,500		2,324,800
Current liabilities	832,500		845,800
Non-current liabilities	539,000		537,200
TOTAL LIABILITIES	1,371,500		1,383,000
Joint ventures' equity	1,041,000		941,800
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,412,500		2,324,800
AECOM's investment in unconsolidated joint ventures	327,500		$ 328,900
Joint ventures summarized financial information
Revenue	416,600	584,300
Cost of revenue	397,700	557,500
Gross profit	18,900	26,800
Net income	$ 17,300	$ 25,400