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Organization and Description of Business	12 Months Ended
Organization and Description of Business	Sep. 27, 2024
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Description of Business	Organization and Description of Business Amentum Holdings, Inc. (collectively with its subsidiaries, “we,” “us,” “our,” “Amentum,” or the “Company”) is a global advanced engineering and technology solutions provider to a broad base of U.S. and allied government agencies, supporting programs of critical national importance across energy and environmental, intelligence, space, defense, civilian and commercial end-markets. We offer a broad reach of capabilities including environment and climate sustainability, intelligence and counter threat solutions, data fusion and analytics, engineering and integration, advanced test, training and readiness, and citizen solutions. As a leading provider of differentiated technology solutions, we have built a repertoire of deep customer knowledge, enabling us to engage our customers across multiple capabilities and markets. On September 27, 2024, the spin-off of the Jacobs Solutions Inc. (“Jacobs”) Critical Mission Solutions business and portions of the Jacobs Divergent Solutions business (and, together with the Critical Mission Solutions business, referred to as the “CMS Business” or “CMS”) merged with Amentum Parent Holdings LLC (collectively, the “Transaction”) with the surviving entity renamed Amentum Holdings, Inc. Amentum's Registration Statement on Form 10 (the “Registration Statement”), filed with the Securities and Exchange Commission (“SEC”) on July 15, 2024, was declared effective on September 18, 2024. Amentum Parent Holdings LLC is the accounting acquirer of CMS for accounting purposes in accordance with accounting principles generally accepted in the United States of America (“GAAP”). Amentum Parent Holdings LLC is considered the Company’s predecessor and the historical financial statements of Amentum Parent Holdings LLC prior to September 27, 2024, are reflected in this Annual Report on Form 10-K as the Company’s historical financial statements. Accordingly, the financial results of the Company prior to September 27, 2024 do not include the financial results of CMS and current and future results will not be comparable to historical results.