Joint Ventures and Variable Interest Entities - Consolidated (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
Total AECOM equity	2,510,672		2,712,470
Noncontrolling interests	115,567		117,107
TOTAL STOCKHOLDERS' EQUITY	2,626,239	$ 3,020,177	2,829,577	$ 3,413,544
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	11,443,018		11,733,954
Revenue	3,266,716	3,313,155
Consolidated Joint Ventures
Financial Information
Current assets	486,000		503,900
Non-current assets	74,400		74,800
TOTAL ASSETS	560,400		578,700
Current liabilities	382,600		400,300
Non-current liabilities	1,500		1,500
TOTAL LIABILITIES	384,100		401,800
Total AECOM equity	69,300		74,000
Noncontrolling interests	107,000		102,900
TOTAL STOCKHOLDERS' EQUITY	176,300		176,900
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	560,400		$ 578,700
Revenue	$ 301,200	$ 189,200