Joint Ventures - Condensed Financial Statements (Details) - USD ($) $ in Millions	Oct. 03, 2025	Sep. 27, 2024	Sep. 29, 2023	Sep. 30, 2022
Variable Interest Entity [Line Items]
Cash and cash equivalents	$ 437	$ 452
Total assets	11,460	11,974
Current liabilities	2,354	1,965
Total liabilities	6,840	7,422
Total Amentum equity	4,505	4,460
Non-controlling interests	115	92
Total shareholders' equity	4,620	4,552	$ 416	$ 706
Total liabilities and shareholders' equity	11,460	11,974
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Cash and cash equivalents	167	160
Current assets	191	322
Non-current assets	0	2
Total assets	358	484
Current liabilities	146	190
Non-current liabilities	0	1
Total liabilities	146	191
Total Amentum equity	153	228
Non-controlling interests	59	65
Total shareholders' equity	212	293
Total liabilities and shareholders' equity	$ 358	$ 484