Segment Information - Schedule of Total Revenues, Segment Operating Profit and Total Asset for Reporting Segment (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Mar. 28, 2025	Mar. 29, 2024	Mar. 28, 2025	Mar. 29, 2024
Revenues from External Customers:
Revenues	$ 2,910,415	$ 2,847,179	$ 5,843,371	$ 5,657,406
Segment Operating Profit:
Total Segment Operating Profit	208,648	183,244	417,066	325,243
Restructuring, Transaction and Other Charges	(23,924)	(42,550)	(53,856)	(85,463)
Amortization of Intangible Assets	(38,040)	(38,476)	(76,701)	(75,407)
Total Other (Expense) Income, net	(152,825)	(39,235)	(308,095)	(78,030)
Earnings from Continuing Operations Before Taxes	55,823	144,009	108,971	247,213
Restructuring charges			25,403
Amentum Holdings, Inc.
Segment Operating Profit:
Restructuring charges	8,400		16,200
Amentum Holdings, Inc. \| Discontinued Operations, Disposed of by Means Other than Sale, Spinoff \| SpinCo Business
Segment Operating Profit:
Equity method investment retained after disposal, mark-to market losses	$ 109,500		$ 254,700
Joint Venture
Segment Operating Profit:
Equity Method Investment, Ownership Percentage	50.00%		50.00%
CH2M HILL Companies, Ltd.
Segment Operating Profit:
Restructuring charges	$ 10,663	35,362	$ 25,403	73,716
CH2M HILL Companies, Ltd. \| Professional Services and Employee Seperation
Segment Operating Profit:
Restructuring charges	10,200		25,100	69,500
Operating Segments
Segment Operating Profit:
Total Segment Operating Profit	270,612	264,270	547,623	486,113
Infrastructure & Advanced Facilities \| Operating Segments
Revenues from External Customers:
Revenues	2,602,753	2,553,212	5,228,961	5,057,438
Segment Operating Profit:
Total Segment Operating Profit	203,265	204,101	413,539	371,489
PA Consulting \| Operating Segments
Revenues from External Customers:
Revenues	307,662	293,967	614,410	599,968
Segment Operating Profit:
Total Segment Operating Profit	67,347	60,169	134,084	114,624
PA Consulting \| Operating Segments \| CH2M HILL Companies, Ltd.
Segment Operating Profit:
Restructuring charges	$ 495	$ 2,984	$ 259	$ 4,159

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