Acquisition and Divestiture - Narrative (Details) - USD ($) $ in Millions			1 Months Ended	12 Months Ended
	Sep. 27, 2024	Sep. 26, 2024	Mar. 31, 2025	Oct. 03, 2025	Sep. 27, 2024	Jun. 26, 2025
Business Combination [Line Items]
Common stock, issued (shares)	243,302,173			243,464,776	243,302,173
Common stock, outstanding (shares)	243,302,173			243,464,776	243,302,173
Debt issuance costs written off					$ 31
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Rapid Solutions
Business Combination [Line Items]
Disposal group, including discontinued operation, consideration						$ 360
Jacobs Solutions Inc. And Shareholders
Business Combination [Line Items]
Percentage of issued and outstanding shares of common stock held	58.50%
Amentum Equityholder
Business Combination [Line Items]
Percentage of issued and outstanding shares of common stock held	37.00%
Critical Mission Solutions, Shareholders
Business Combination [Line Items]
Percentage of outstanding shares issued and outstanding, released from escrow			3.50%
Sponsor Stockholder
Business Combination [Line Items]
Percentage of outstanding shares issued and outstanding, released from escrow			1.00%
Jacobs Solutions Inc. | CMS
Business Combination [Line Items]
Payments of dividends		$ 911
CMS
Business Combination [Line Items]
Percentage of issued and outstanding shares held in escrow	4.50%
Transaction costs					79
Business combination, goodwill, expected tax deductible, amount	$ 737				737
Intangible assets	$ 1,860			$ 1,805	$ 1,860
CMS | Backlog
Business Combination [Line Items]
Intangible assets				$ 275
Fair value attributed to intangible assets				1 year
CMS | Customer relationship intangible assets
Business Combination [Line Items]
Intangible assets				$ 1,530
Fair value attributed to intangible assets				14 years
CMS | Selling, general and administrative expenses
Business Combination [Line Items]
Transaction costs				$ 48