CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Total Shareholders' Equity Attributable to Amentum Holdings, Inc.	Common Stock	Additional Paid-in Capital	Retained Deficit	Accumulated Other Comprehensive Income (Loss)	Non-controlling Interests
Beginning balance (shares) at Sep. 30, 2022			0
Beginning balance at Sep. 30, 2022	$ 706	$ 633	$ 0	$ 755	$ (131)	$ 9	$ 73
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) including non-controlling interests	(321)	(314)			(314)		(7)
Other comprehensive income (loss), net of tax	39	39				39
Acquisition of remaining interest in consolidated joint ventures	0	14		14			(14)
Capital contribution from non-controlling interest	13						13
Distributions to non-controlling interests	(24)						(24)
Stock-based compensation and other	3	3		3
Ending balance (shares) at Sep. 29, 2023			0
Ending balance at Sep. 29, 2023	416	375	$ 0	772	(445)	48	41
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) including non-controlling interests	(83)	(82)			(82)		(1)
Other comprehensive income (loss), net of tax	(25)	(25)				(25)
Acquisition of CMS (in shares)			243,000,000
Acquisition of CMS	4,000	3,937	$ 2	3,935			63
Capital contribution from non-controlling interest	235	235		235
Distributions to non-controlling interests	(6)						(6)
Stock-based compensation and other	$ 15	20		20			(5)
Ending balance (shares) at Sep. 27, 2024	243,302,173		243,000,000
Ending balance at Sep. 27, 2024	$ 4,552	4,460	$ 2	4,962	(527)	23	92
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) including non-controlling interests	59	66			66		(7)
Other comprehensive income (loss), net of tax	17	17				17
Measurement period adjustments	1	(63)		(63)			64
Issuances of common stock	4	4		4
Capital contribution from non-controlling interest	3			0			3
Distributions to non-controlling interests	(35)						(35)
Stock-based compensation and other	$ 19	21		21			(2)
Ending balance (shares) at Oct. 03, 2025	243,464,776		243,000,000
Ending balance at Oct. 03, 2025	$ 4,620	$ 4,505	$ 2	$ 4,924	$ (461)	$ 40	$ 115