Joint Ventures and Variable Interest Entities - Unconsolidated (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Financial Information
Current assets	$ 6,173,543				$ 7,530,366				$ 6,173,543	$ 7,530,366
TOTAL ASSETS	11,733,954				12,998,951				11,733,954	12,998,951
Current liabilities	5,521,715				6,090,454				5,521,715	6,090,454
TOTAL LIABILITIES	8,904,377				9,585,407				8,904,377	9,585,407
Joint ventures' equity	2,829,577				3,413,544				2,829,577	3,413,544	$ 3,899,350	$ 4,278,374
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	11,733,954				12,998,951				11,733,954	12,998,951
AECOM's investment in unconsolidated joint ventures	328,906				297,595				328,906	297,595
Joint ventures summarized financial information
Revenue	3,353,800	$ 3,408,400	$ 3,265,500	$ 3,313,200	3,569,000	$ 3,189,700	$ 3,245,700	$ 3,235,600	13,340,852	13,239,976	13,642,455
Cost of revenue	3,136,600	3,206,800	3,070,300	3,128,800	3,379,100	3,004,600	3,076,900	3,069,800	12,542,431	12,530,416	13,030,800
Gross profit	217,200	201,600	195,200	184,400	189,900	185,100	168,800	165,800	798,421	709,560	611,655
Net income	106,300	$ 18,400	$ 45,300	$ 33,000	(223,900)	$ 94,100	$ (77,000)	$ 53,100	202,980	(153,741)	$ (183,990)
Unconsolidated Joint Ventures
Financial Information
Current assets	1,323,200				1,374,300				1,323,200	1,374,300
Non-current assets	1,001,600				465,800				1,001,600	465,800
TOTAL ASSETS	2,324,800				1,840,200				2,324,800	1,840,200
Current liabilities	845,800				953,400				845,800	953,400
Non-current liabilities	537,200				58,900				537,200	58,900
TOTAL LIABILITIES	1,383,000				1,012,300				1,383,000	1,012,300
Joint ventures' equity	941,800				827,900				941,800	827,900
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	2,324,800				1,840,200				2,324,800	1,840,200
AECOM's investment in unconsolidated joint ventures	$ 328,900				$ 297,600				328,900	297,600
Joint ventures summarized financial information
Revenue									2,096,500	3,058,900
Cost of revenue									2,051,200	2,993,100
Gross profit									45,300	65,800
Net income									$ 37,000	$ 59,800