Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Summarized financial information concerning the Company's reportable segments
Revenue	$ 3,213,658	$ 3,265,573	$ 6,480,374	$ 6,578,728
Gross profit	209,952	195,259	410,156	379,629
Equity in earnings of joint ventures	11,919	7,157	19,869	15,358
General and administrative expenses	(37,098)	(36,007)	(73,599)	(74,367)
Restructuring costs	(73,292)	(8,746)	(76,663)	(21,784)
Operating income	111,481	157,663	279,763	298,836
Segment assets	11,196,063		11,196,063		$ 11,733,954
Total
Summarized financial information concerning the Company's reportable segments
Revenue	3,213,700	3,265,500	6,480,400	6,578,700
Gross profit	210,000	195,200	410,200	379,600
Equity in earnings of joint ventures	12,000	7,200	19,900	15,400
General and administrative expenses	(37,200)	(36,000)	(73,600)	(74,400)
Restructuring costs	(73,300)	(8,800)	(76,700)	(21,800)
Operating income	$ 111,500	$ 157,600	$ 279,800	$ 298,800
Gross profit as a % of revenue	6.50%	6.00%	6.30%	5.80%
Corporate
Summarized financial information concerning the Company's reportable segments
General and administrative expenses	$ (34,600)	$ (34,500)	$ (68,000)	$ (71,000)
Restructuring costs	(73,300)	(8,800)	(76,700)	(21,800)
Operating income	(107,900)	(43,300)	(144,700)	(92,800)
Segment assets	1,147,100		1,147,100		1,390,900
Americas
Summarized financial information concerning the Company's reportable segments
Revenue	2,399,900	2,468,300	4,863,400	5,025,600
Gross profit	160,400	151,700	310,400	296,700
Equity in earnings of joint ventures	3,200	3,000	6,400	4,400
Operating income	$ 163,600	$ 154,700	$ 316,800	$ 301,100
Gross profit as a % of revenue	6.70%	6.10%	6.40%	5.90%
Segment assets	$ 7,134,900		$ 7,134,900		7,204,600
International
Summarized financial information concerning the Company's reportable segments
Revenue	813,300	$ 796,500	1,615,700	$ 1,552,100
Gross profit	49,100	42,800	98,500	81,900
Equity in earnings of joint ventures	5,700	3,000	9,300	6,000
Operating income	$ 54,800	$ 45,800	$ 107,800	$ 87,900
Gross profit as a % of revenue	6.00%	5.40%	6.10%	5.30%
Segment assets	$ 2,593,100		$ 2,593,100		2,764,500
AECOM Capital
Summarized financial information concerning the Company's reportable segments
Revenue	500	$ 700	1,300	$ 1,000
Gross profit	500	700	1,300	1,000
Equity in earnings of joint ventures	3,100	1,200	4,200	5,000
General and administrative expenses	(2,600)	(1,500)	(5,600)	(3,400)
Operating income	1,000	$ 400	(100)	$ 2,600
Segment assets	$ 236,600		$ 236,600		$ 234,600