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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% |  |

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