Segment Information - Schedule of Total Revenues, Segment Operating Profit and Total Asset for Reporting Segment (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Mar. 13, 2025	Sep. 26, 2025	Sep. 27, 2024	Sep. 26, 2025	Sep. 27, 2024	Sep. 29, 2023
Segment Reporting Information [Line Items]
Revenues from External Customers				$ 12,029,783	$ 11,500,941	$ 10,851,420
Direct cost of contracts				(9,044,849)	(8,668,185)	(8,140,560)
Selling, general and administrative expenses				(2,121,300)	(2,140,320)	(2,034,376)
Operating Profit				863,634	692,436	676,484
Restructuring, Transaction and Other Charges				(162,896)	(192,522)	(146,891)
Amortization of Intangible Assets				(155,517)	(152,667)	(147,230)
Total Other (Expense) Income, net				(320,157)	84,850	(155,509)
Earnings from Continuing Operations Before Taxes				543,477	777,286	520,975
Restructuring charges				61,316
Equity method investment retained after disposal, mark-to market (losses) gains				227,300	(186,900)
Transition services agreement, income				40,500
Loss on extinguishment of debt				20,510	0	0
Recognized impairment charges						46,700
Interest Rate Swap
Segment Reporting Information [Line Items]
Derivative, gain on derivative		$ 35,200	$ 35,200		35,200
Revolving Credit Facility | 2021 Term Loan Facility
Segment Reporting Information [Line Items]
Loss on extinguishment of debt	$ (20,500)
Amentum Holdings, Inc.
Segment Reporting Information [Line Items]
Restructuring charges				26,000
Amentum Holdings, Inc. | Discontinued Operations, Spinoff | SpinCo Business
Segment Reporting Information [Line Items]
Equity method investment retained after disposal, mark-to market (losses) gains				227,300	(186,900)
Transition services agreement, income				$ 40,500
Joint Venture
Segment Reporting Information [Line Items]
Ownership percentage		50.00%		50.00%
CH2M HILL Companies, Ltd.
Segment Reporting Information [Line Items]
Restructuring charges				$ 61,316	134,911	126,219
CH2M HILL Companies, Ltd. | Professional Services and Employee Separation Costs
Segment Reporting Information [Line Items]
Restructuring charges				58,800	163,400	61,100
CH2M HILL Companies, Ltd. | Employee Separation Costs
Segment Reporting Information [Line Items]
Restructuring charges						14,300
CH2M HILL Companies, Ltd. | Professional Services and Employee Separation
Segment Reporting Information [Line Items]
Restructuring charges				58,800
PA Consulting Group Limited
Segment Reporting Information [Line Items]
Allocated share-based compensation expense				75,300	13,400
PA Consulting | CH2M HILL Companies, Ltd. | Employee Separation Costs
Segment Reporting Information [Line Items]
Restructuring charges				2,200	6,400	14,300
Operating Segments
Segment Reporting Information [Line Items]
Revenues from External Customers				12,029,783	11,500,941	10,851,420
Direct cost of contracts				(9,044,849)	(8,668,185)	(8,140,560)
Selling, general and administrative expenses				(1,802,887)	(1,795,131)	(1,740,255)
Operating Profit				1,182,047	1,037,625	970,605
Operating Segments | Employee Separation Costs
Segment Reporting Information [Line Items]
Restructuring charges					6,400
Operating Segments | Infrastructure & Advanced Facilities
Segment Reporting Information [Line Items]
Revenues from External Customers				10,764,206	10,323,255	9,693,276
Direct cost of contracts				(8,228,935)	(7,915,256)	(7,395,838)
Selling, general and administrative expenses				(1,631,723)	(1,609,624)	(1,563,836)
Operating Profit				903,548	798,375	733,602
Operating Segments | Infrastructure & Advanced Facilities | CH2M HILL Companies, Ltd.
Segment Reporting Information [Line Items]
Restructuring charges				59,092	128,529	111,513
Operating Segments | PA Consulting
Segment Reporting Information [Line Items]
Revenues from External Customers				1,265,577	1,177,686	1,158,144
Direct cost of contracts				(815,914)	(752,929)	(744,722)
Selling, general and administrative expenses				(171,164)	(185,507)	(176,419)
Operating Profit				278,499	239,250	237,003
Operating Segments | PA Consulting | CH2M HILL Companies, Ltd.
Segment Reporting Information [Line Items]
Restructuring charges				$ 2,224	$ 6,382	14,706
Segment Reporting | Other Expense
Segment Reporting Information [Line Items]
Compensation expense, cost reductions						15,000
Compensation expense, employee benefit program costs						41,000
Technology platform and personnel and corporate overhead costs						$ 26,000