Reportable Segments and Geographic Information - Schedule of Financial Information Concerning the Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Revenue	$ 16,139,622	$ 16,105,498	$ 14,378,461
Equity in earnings (losses) of joint ventures	27,013	2,124	(279,352)
Earnings before income taxes and amortization	1,181,300	1,046,400	642,400
Total
Segment Reporting Information [Line Items]
Revenue	16,139,600	16,105,500	14,378,500
Subcontractor and other direct costs	(8,566,800)	(8,940,500)	(7,675,700)
Employee compensation expense	(5,375,500)	(5,166,100)	(4,870,300)
Equity in earnings (losses) of joint ventures	27,000	2,100	(279,400)
Other segment items	(1,043,000)	(954,600)	(910,700)
Earnings before income taxes and amortization	1,181,300	1,046,400	642,400
Operating Segments | Americas
Segment Reporting Information [Line Items]
Revenue	12,525,900	12,485,700	10,975,700
Subcontractor and other direct costs	(7,973,700)	(8,281,100)	(7,056,700)
Employee compensation expense	(3,089,400)	(2,929,000)	(2,729,900)
Equity in earnings (losses) of joint ventures	15,800	15,500	14,800
Other segment items	(609,700)	(514,900)	(482,300)
Earnings before income taxes and amortization	868,900	776,200	721,600
Operating Segments | International
Segment Reporting Information [Line Items]
Revenue	3,613,200	3,618,400	3,402,100
Subcontractor and other direct costs	(593,100)	(659,400)	(619,000)
Employee compensation expense	(2,286,100)	(2,234,400)	(2,134,500)
Equity in earnings (losses) of joint ventures	11,800	13,500	9,700
Other segment items	(424,400)	(427,400)	(421,700)
Earnings before income taxes and amortization	321,400	310,700	236,600
Operating Segments | AECOM Capital
Segment Reporting Information [Line Items]
Revenue	500	1,400	700
Subcontractor and other direct costs	0	0	0
Employee compensation expense	0	(2,700)	(5,900)
Equity in earnings (losses) of joint ventures	(600)	(26,900)	(303,900)
Other segment items	(8,900)	(12,300)	(6,700)
Earnings before income taxes and amortization	$ (9,000)	$ (40,500)	$ (315,800)