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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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