Condensed Consolidating Financial Information - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
CURRENT ASSETS:
Total cash and cash equivalents	$ 1,080,354	$ 886,733
Accounts receivable-net	3,517,072	3,307,851
Prepaid expenses and other current assets	627,550	585,152
Current assets held for sale		59,800
Income taxes receivable	49,089	126,816
TOTAL CURRENT ASSETS	7,534,645	7,127,322
PROPERTY AND EQUIPMENT-NET	559,399	614,062
DEFERRED TAX ASSETS-NET	245,331	159,396
INVESTMENTS IN UNCONSOLIDATED JOINT VENTURES	405,225	310,661
GOODWILL	5,275,281	5,921,116	$ 5,992,900
INTANGIBLE ASSETS-NET	233,018	319,892
OTHER NON-CURRENT ASSETS	208,692	228,682
TOTAL ASSETS	14,461,591	14,681,131
CURRENT LIABILITIES:
Short-term debt	47,835	8,353
Accounts payable	2,954,719	2,726,047
Accrued expenses and other current liabilities	2,390,418	2,267,046
Income taxes payable	59,541	39,802
Contract liabilities	939,891	931,431
Current liabilities held for sale		22,300
Current portion of long-term debt	69,350	134,698
TOTAL CURRENT LIABILITIES	6,461,754	6,129,677
DEFERRED TAX LIABILITY-NET	4,292	47,273
LONG-TERM DEBT	3,285,755	3,483,746
TOTAL LIABILITIES	10,562,241	10,402,757
TOTAL AECOM STOCKHOLDERS' EQUITY	3,690,576	4,092,780
Noncontrolling interests	208,774	185,594
TOTAL STOCKHOLDERS' EQUITY	3,899,350	4,278,374	4,214,686	$ 3,552,489
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	14,461,591	14,681,131
Eliminations
CURRENT ASSETS:
Intercompany receivable	(1,504,600)	(1,193,900)
TOTAL CURRENT ASSETS	(1,504,600)	(1,193,900)
DEFERRED TAX ASSETS-NET	(95,200)	(124,600)
INVESTMENTS IN CONSOLIDATED SUBSIDIARIES	(7,352,000)	(8,276,100)
TOTAL ASSETS	(8,951,800)	(9,594,600)
CURRENT LIABILITIES:
Intercompany payable	(1,639,500)	(1,353,200)
TOTAL CURRENT LIABILITIES	(1,639,500)	(1,353,200)
DEFERRED TAX LIABILITY-NET	(95,200)	(124,700)
NOTE PAYABLE INTERCOMPANY-NON CURRENT	(1,340,100)	(1,288,400)
TOTAL LIABILITIES	(3,074,800)	(2,766,300)
TOTAL AECOM STOCKHOLDERS' EQUITY	(5,877,000)	(6,828,300)
TOTAL STOCKHOLDERS' EQUITY	(5,877,000)	(6,828,300)
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	(8,951,800)	(9,594,600)
Parent | Reportable Legal Entities
CURRENT ASSETS:
Total cash and cash equivalents	129,300	22,000
Intercompany receivable	1,164,700	951,100
Prepaid expenses and other current assets	52,500	52,900
Income taxes receivable	13,700	84,600
TOTAL CURRENT ASSETS	1,360,200	1,110,600
PROPERTY AND EQUIPMENT-NET	193,000	202,600
DEFERRED TAX ASSETS-NET	152,800	134,000
INVESTMENTS IN CONSOLIDATED SUBSIDIARIES	5,740,800	6,364,100
INVESTMENTS IN UNCONSOLIDATED JOINT VENTURES	9,900	13,400
OTHER NON-CURRENT ASSETS	33,100	49,900
TOTAL ASSETS	7,489,800	7,874,600
CURRENT LIABILITIES:
Short-term debt	21,800	8,400
Accounts payable	50,200	53,600
Accrued expenses and other current liabilities	108,000	58,800
Income taxes payable	23,600	10,400
Intercompany payable	116,100	105,500
Billings in excess of costs on uncompleted contracts		1,500
Current portion of long-term debt	12,600	43,300
TOTAL CURRENT LIABILITIES	332,300	281,500
OTHER LONG-TERM LIABILITIES	130,700	131,600
NOTE PAYABLE INTERCOMPANY-NON CURRENT	872,600	800,900
LONG-TERM DEBT	2,468,900	2,627,800
TOTAL LIABILITIES	3,804,500	3,841,800
TOTAL AECOM STOCKHOLDERS' EQUITY	3,685,300	4,032,800
TOTAL STOCKHOLDERS' EQUITY	3,685,300	4,032,800
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	7,489,800	7,874,600
Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS:
Total cash and cash equivalents	315,600	270,900
Accounts receivable and contract assets-net	2,651,800	2,544,700
Intercompany receivable	163,900	84,900
Prepaid expenses and other current assets	270,100	331,600
TOTAL CURRENT ASSETS	3,401,400	3,232,100
PROPERTY AND EQUIPMENT-NET	179,100	217,300
DEFERRED TAX ASSETS-NET	45,600
INVESTMENTS IN CONSOLIDATED SUBSIDIARIES	1,611,200	1,912,000
INVESTMENTS IN UNCONSOLIDATED JOINT VENTURES	41,600	49,600
GOODWILL	3,193,400	3,392,700
INTANGIBLE ASSETS-NET	172,300	218,600
OTHER NON-CURRENT ASSETS	43,500	45,600
TOTAL ASSETS	8,688,100	9,067,900
CURRENT LIABILITIES:
Accounts payable	1,946,100	1,616,700
Accrued expenses and other current liabilities	1,012,100	1,035,600
Intercompany payable	873,900	830,800
Billings in excess of costs on uncompleted contracts		316,100
Contract liabilities	318,800
Current portion of long-term debt	14,800	27,000
TOTAL CURRENT LIABILITIES	4,165,700	3,826,200
OTHER LONG-TERM LIABILITIES	288,200	249,000
DEFERRED TAX LIABILITY-NET		63,100
LONG-TERM DEBT	290,100	291,400
TOTAL LIABILITIES	4,744,000	4,429,700
TOTAL AECOM STOCKHOLDERS' EQUITY	3,944,100	4,638,200
TOTAL STOCKHOLDERS' EQUITY	3,944,100	4,638,200
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	8,688,100	9,067,900
Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS:
Total cash and cash equivalents	635,500	593,800
Accounts receivable and contract assets-net	3,125,900	2,924,100
Intercompany receivable	176,000	157,900
Prepaid expenses and other current assets	304,800	200,700
Current assets held for sale		59,800
Income taxes receivable	35,400	42,200
TOTAL CURRENT ASSETS	4,277,600	3,978,500
PROPERTY AND EQUIPMENT-NET	187,300	194,200
DEFERRED TAX ASSETS-NET	142,100	150,000
INVESTMENTS IN UNCONSOLIDATED JOINT VENTURES	353,700	247,700
GOODWILL	2,081,900	2,528,400
INTANGIBLE ASSETS-NET	60,700	101,300
OTHER NON-CURRENT ASSETS	132,200	133,100
TOTAL ASSETS	7,235,500	7,333,200
CURRENT LIABILITIES:
Short-term debt	26,000
Accounts payable	958,400	1,055,700
Accrued expenses and other current liabilities	1,270,300	1,172,700
Income taxes payable	36,000	29,400
Intercompany payable	649,500	416,900
Billings in excess of costs on uncompleted contracts		613,800
Contract liabilities	621,100
Current liabilities held for sale		22,300
Current portion of long-term debt	42,000	64,400
TOTAL CURRENT LIABILITIES	3,603,300	3,375,200
OTHER LONG-TERM LIABILITIES	391,400	361,500
DEFERRED TAX LIABILITY-NET	99,500	108,900
NOTE PAYABLE INTERCOMPANY-NON CURRENT	467,500	487,500
LONG-TERM DEBT	526,800	564,500
TOTAL LIABILITIES	5,088,500	4,897,600
TOTAL AECOM STOCKHOLDERS' EQUITY	1,938,200	2,250,100
Noncontrolling interests	208,800	185,500
TOTAL STOCKHOLDERS' EQUITY	2,147,000	2,435,600
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	7,235,500	7,333,200
Total
CURRENT ASSETS:
Total cash and cash equivalents	1,080,400	886,700
Accounts receivable and contract assets-net	5,777,700	5,468,800
Accounts receivable-net	3,517,100	3,307,800
Prepaid expenses and other current assets	627,400	585,200
Current assets held for sale		59,800
Income taxes receivable	49,100	126,800
TOTAL CURRENT ASSETS	7,534,600	7,127,300
PROPERTY AND EQUIPMENT-NET	559,400	614,100
DEFERRED TAX ASSETS-NET	245,300	159,400
INVESTMENTS IN UNCONSOLIDATED JOINT VENTURES	405,200	310,700
GOODWILL	5,275,300	5,921,100
INTANGIBLE ASSETS-NET	233,000	319,900
OTHER NON-CURRENT ASSETS	208,800	228,600
TOTAL ASSETS	14,461,600	14,681,100	$ 14,397,000
CURRENT LIABILITIES:
Short-term debt	47,800	8,400
Accounts payable	2,954,700	2,726,000
Accrued expenses and other current liabilities	2,390,400	2,267,100
Income taxes payable	59,600	39,800
Billings in excess of costs on uncompleted contracts		931,400
Contract liabilities	939,900
Current liabilities held for sale		22,300
Current portion of long-term debt	69,400	134,700
TOTAL CURRENT LIABILITIES	6,461,800	6,129,700
OTHER LONG-TERM LIABILITIES	810,300	742,100
DEFERRED TAX LIABILITY-NET	4,300	47,300
LONG-TERM DEBT	3,285,800	3,483,700
TOTAL LIABILITIES	10,562,200	10,402,800
TOTAL AECOM STOCKHOLDERS' EQUITY	3,690,600	4,092,800
Noncontrolling interests	208,800	185,500
TOTAL STOCKHOLDERS' EQUITY	3,899,400	4,278,300
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 14,461,600	$ 14,681,100