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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)

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