Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Summarized financial information concerning the Company's reportable segments
Revenue	$ 3,266,716	$ 3,313,155
Gross profit	200,204	184,370
Equity in earnings of joint ventures	7,950	8,201
General and administrative expenses	(36,501)	(38,360)
Restructuring costs	(3,371)	(13,038)
Operating income	168,282	141,173
Segment assets	11,443,018		$ 11,733,954
Total
Summarized financial information concerning the Company's reportable segments
Revenue	3,266,700	3,313,200
Gross profit	200,200	184,400
Equity in earnings of joint ventures	7,900	8,200
General and administrative expenses	(36,400)	(38,400)
Restructuring costs	(3,400)	(13,000)
Operating income	$ 168,300	$ 141,200
Gross profit as a % of revenue	6.10%	5.60%
Corporate
Summarized financial information concerning the Company's reportable segments
General and administrative expenses	$ (33,400)	$ (36,500)
Restructuring costs	(3,400)	(13,000)
Operating income	(36,800)	(49,500)
Segment assets	1,320,500		1,390,900
Americas
Summarized financial information concerning the Company's reportable segments
Revenue	2,463,500	2,557,300
Gross profit	150,000	145,000
Equity in earnings of joint ventures	3,200	1,400
Operating income	$ 153,200	$ 146,400
Gross profit as a % of revenue	6.10%	5.70%
Segment assets	$ 7,090,100		7,204,600
International
Summarized financial information concerning the Company's reportable segments
Revenue	802,400	$ 755,600
Gross profit	49,400	39,100
Equity in earnings of joint ventures	3,600	3,000
Operating income	$ 53,000	$ 42,100
Gross profit as a % of revenue	6.20%	5.20%
Segment assets	$ 2,691,300		2,764,500
AECOM Capital
Summarized financial information concerning the Company's reportable segments
Revenue	800	$ 300
Gross profit	800	300
Equity in earnings of joint ventures	1,100	3,800
General and administrative expenses	(3,000)	(1,900)
Operating income	(1,100)	$ 2,200
Segment assets	$ 233,300		$ 234,600