| Document and Entity Information | Sep. 27, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Sep. 27, 2024 |
| Current Fiscal Year End Date | --09-27 |
| Entity File Number | 001-42176 |
| Entity Registrant Name | Amentum Holdings, Inc. |
| Entity Central Index Key | 0002011286 |
| 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, $0.01 par value |
| Trading Symbol | AMTM |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
