Acquisition and Divestiture - Schedule of Total Consideration Exchange (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended	12 Months Ended
	Sep. 27, 2024	Mar. 31, 2025	Oct. 03, 2025	Sep. 27, 2024	Sep. 29, 2023
Business Combination [Line Items]
Per share price of Amentum Holdings, Inc. common stock (usd per share)	$ 25.67			$ 25.67
Equity method investments	$ 123		$ 196	$ 123
Gain on acquisition of controlling interest			0	69	$ 0
Joint Venture Prior To Acquisition
Business Combination [Line Items]
Equity method investments	$ 170			$ 170
Jacobs Solutions Inc. And Shareholders
Business Combination [Line Items]
Percentage of issued and outstanding shares of common stock held	58.50%
Sponsor Stockholder
Business Combination [Line Items]
Percentage of outstanding shares issued and outstanding, released from escrow		1.00%
Critical Mission Solutions, Shareholders
Business Combination [Line Items]
Percentage of outstanding shares issued and outstanding, released from escrow		3.50%
CMS
Business Combination [Line Items]
Total consideration exchanged			$ (7)
Shares of Amentum Holdings, Inc. common stock issued to CMS shareholders (in shares)	142
Fair value of common stock issued to CMS shareholders	$ 3,654
Preliminary fair value of additional equity consideration issued to CMS shareholders	218
Final working capital settlement	70
Other consideration	6
Fair value of consideration transferred	3,948
Fair value of previously held equity interest	84
Total consideration	4,032
Noncontrolling interest ownership percentage			50.00%
Gain on acquisition of controlling interest	69
Receivable balance settled in acquisition	$ 1
CMS | Third Party
Business Combination [Line Items]
Noncontrolling interest ownership percentage			10.00%
CMS | Critical Mission Solutions, Shareholders
Business Combination [Line Items]
Noncontrolling interest ownership percentage			40.00%