Corporate Expense Allocation (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 27, 2025	Sep. 28, 2024	Sep. 30, 2023
Corporate Expense Allocation [Abstract]
Corporate expense allocation	$ 3	$ 21	$ 26

Amount of general corporate expenses, including information technology, accounting, legal, human resources, and other services, allocated to subsidiary as determined by management.

No definition available.

Name:	magn_CorporateExpenseAllocation
Namespace Prefix:	magn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration