Segment Information - Adjusted EBITDA (Details) - USD ($) $ in Millions	12 Months Ended
	Oct. 03, 2025	Sep. 27, 2024	Sep. 29, 2023
Segment Reporting Information [Line Items]
Revenues	$ 14,393	$ 8,388	$ 7,865
Cost of revenues	(12,880)	(7,590)	(7,083)
Other segment expenses	(409)	(180)	(186)
Adjusted EBITDA attributable to Amentum Holdings, Inc.	1,104	618	596
Depreciation expense	(40)	(23)	(27)
Amortization of intangibles	(479)	(228)	(298)
Interest expense and other, net	(353)	(438)	(397)
Non-controlling interests	(7)	(1)	(7)
Acquisition, transaction and integration costs	(85)	(62)	(39)
Non-cash GAAP expense (gain)	0	69	(186)
Loss on extinguishment of debt	(12)	(45)	0
Utilization of fair market value adjustments	8	5	21
Stock-based compensation	(21)	(18)	(3)
Income (loss) before income taxes	115	(123)	(340)
Provision for income taxes	(56)	40	19
Net income (loss) including non-controlling interests	59	(83)	(321)
Net income (loss) attributable to common shareholders	66	(82)	(314)
DS
Segment Reporting Information [Line Items]
Revenues	5,543	1,981	1,899
Cost of revenues	(4,924)	(1,761)	(1,659)
Other segment expenses	(182)	(61)	(81)
Adjusted EBITDA attributable to Amentum Holdings, Inc.	437	159	159
GES
Segment Reporting Information [Line Items]
Revenues	8,850	6,407	5,966
Cost of revenues	(7,956)	(5,829)	(5,424)
Other segment expenses	(227)	(119)	(105)
Adjusted EBITDA attributable to Amentum Holdings, Inc.	$ 667	$ 459	$ 437