Reportable Segments - Schedule of Financial Information Concerning the Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenue	$ 4,014,152	$ 3,899,920
Gross profit	268,404	243,970
Equity in earnings (losses) of joint ventures	9,553	(28,941)
General and administrative expenses	(40,459)	(35,724)
Restructuring costs	0	(16,180)
Operating income (loss)	237,498	163,125
Total assets	11,818,827		$ 12,061,669
Total
Segment Reporting Information [Line Items]
Revenue	4,014,200	3,899,900
Gross profit	268,400	244,000
Equity in earnings (losses) of joint ventures	9,600	(29,000)
General and administrative expenses	(40,500)	(35,700)
Restructuring costs		(16,200)
Operating income (loss)	$ 237,500	$ 163,100
Gross profit as a % of revenue	6.70%	6.30%
Operating Segments | Americas
Segment Reporting Information [Line Items]
Revenue	$ 3,112,000	$ 3,038,700
Gross profit	190,200	171,000
Equity in earnings (losses) of joint ventures	5,500	3,600
General and administrative expenses	0	0
Restructuring costs		0
Operating income (loss)	$ 195,700	$ 174,600
Gross profit as a % of revenue	6.10%	5.60%
Total assets	$ 7,832,700		7,988,100
Operating Segments | International
Segment Reporting Information [Line Items]
Revenue	902,000	$ 861,000
Gross profit	78,000	72,800
Equity in earnings (losses) of joint ventures	2,900	4,300
General and administrative expenses	0	0
Restructuring costs		0
Operating income (loss)	$ 80,900	$ 77,100
Gross profit as a % of revenue	8.60%	8.50%
Total assets	$ 2,610,300		2,734,500
Operating Segments | AECOM Capital
Segment Reporting Information [Line Items]
Revenue	200	$ 200
Gross profit	200	200
Equity in earnings (losses) of joint ventures	1,200	(36,900)
General and administrative expenses	(2,400)	(2,400)
Restructuring costs		0
Operating income (loss)	$ (1,000)	$ (39,100)
Gross profit as a % of revenue	0.00%	0.00%
Total assets	$ 53,600		53,200
Corporate
Segment Reporting Information [Line Items]
Revenue	0	$ 0
Gross profit	0	0
Equity in earnings (losses) of joint ventures	0	0
General and administrative expenses	(38,100)	(33,300)
Restructuring costs		(16,200)
Operating income (loss)	$ (38,100)	$ (49,500)
Gross profit as a % of revenue	0.00%	0.00%
Total assets	$ 1,255,100		$ 1,208,700