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Cover - USD ($)
 12 Months Ended
Sep. 27, 2024
Dec. 06, 2024
Cover [Abstract]    
Document Type 10-K  
Document Annual Report true  
Document Period End Date Sep. 27, 2024  
Current Fiscal Year End Date --09-27  
Document Transition Report false  
Entity File Number 001-42176  
Registrant Name Amentum Holdings, Inc.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 99-0622272  
Entity Address, Address Line One 4800 Westfields Blvd., Suite #400  
Entity Address, City or Town Chantilly  
Entity Address, State or Province VA  
Entity Address, Postal Zip Code 20151  
City Area Code 703  
Local Phone Number 579-0410  
Title of 12(b) Security Common Stock, par value $0.01 per share  
Trading Symbol AMTM  
Security Exchange Name NYSE  
Entity Well-known Seasoned Issuer No  
Entity Voluntary Filers No  
Entity Current Reporting Status No  
Entity Interactive Data Current Yes  
Entity Filer Category Non-accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
ICFR Auditor Attestation Flag false  
Document Financial Statement Error Correction [Flag] false  
Entity Shell Company false  
Entity Public Float   $ 5,637,313,295
Entity Common Stock, Shares Outstanding   243,302,257
Documents Incorporated by Reference
Part III of this Form 10-K incorporates by reference certain information from the Registrant’s Proxy Statement to be filed with the Securities Exchange Commission (SEC) pursuant to Regulation 14A for the 2025 Annual Meeting of Stockholders.
  
Central Index Key 0002011286  
Amendment Flag false  
Document Fiscal Year Focus 2024  
Document Fiscal Period Focus FY