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Revenue Recognition - Schedule of Revenues Disaggregated by Revenue Sources (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Total revenue	$ 16,139,622	$ 16,105,498	$ 14,378,461
Americas
Disaggregation of Revenue [Line Items]
Total revenue	12,526,400	12,487,000	10,976,400
Europe, Middle East, India, Africa
Disaggregation of Revenue [Line Items]
Total revenue	2,153,300	2,141,500	1,937,300
Asia-Australia-Pacific
Disaggregation of Revenue [Line Items]
Total revenue	1,459,900	1,477,000	1,464,800
Cost reimbursable
Disaggregation of Revenue [Line Items]
Total revenue	6,196,100	6,361,400	6,128,800
Guaranteed maximum price
Disaggregation of Revenue [Line Items]
Total revenue	5,960,700	6,030,000	4,887,700
Fixed price
Disaggregation of Revenue [Line Items]
Total revenue	$ 3,982,800	$ 3,714,100	$ 3,362,000

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