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Acquisitions - Narrative (Details) - USD ($) $ in Millions			12 Months Ended
	Sep. 27, 2024	Sep. 26, 2024	Sep. 27, 2024	Sep. 29, 2023	Sep. 30, 2022
Business Acquisition [Line Items]
Common stock, issued (shares)	243,302,173		243,302,173	0
Common stock, outstanding (shares)	243,302,173		243,302,173	0
Loss on extinguishment of debt			$ 45	$ 0	$ 32
Jacobs Solutions Inc. And Shareholders
Business Acquisition [Line Items]
Percentage of issued and outstanding shares of common stock held	58.50%
Amentum Equityholder
Business Acquisition [Line Items]
Percentage of issued and outstanding shares of common stock held	37.00%
Jacobs Solutions Inc. | CMS
Business Acquisition [Line Items]
Payments of dividends		$ 911
CMS
Business Acquisition [Line Items]
Percentage of issued and outstanding shares held in escrow	4.50%
Transaction costs			79
Loss on extinguishment of debt			31
Intangible assets	$ 1,860		1,860
CMS | Backlog
Business Acquisition [Line Items]
Intangible assets	270		$ 270
Fair value attributed to intangible assets			1 year
CMS | Customer Relationships
Business Acquisition [Line Items]
Intangible assets	$ 1,590		$ 1,590
Fair value attributed to intangible assets			14 years
CMS | Selling, General and Administrative Expenses
Business Acquisition [Line Items]
Transaction costs			$ 48

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