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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 |  |

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