Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482130/360-10-45-4

+ Details

Name:	us-gaap_AssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of long-term debt instruments that have contractual principal amounts and for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 28
-Subparagraph (d)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-28

+ Details

Name:	us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The cash inflow for a financial contract that meets the hedge criteria as either a cash flow hedge, fair value hedge, or hedge of a net investment in a foreign operation.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 12
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 27
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-27

+ Details

Name:	us-gaap_ProceedsFromHedgeInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CurrencySwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=magn_CrossCurrencySwapMaturingNovember2027Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=magn_CrossCurrencySwapMaturingNovember2029Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Instruments and Fair Value Measurements - Narrative (Details) € in Millions, $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 29, 2025 EUR (€)	Mar. 29, 2025 USD ($)	Mar. 30, 2024 USD ($)	Sep. 28, 2024 USD ($)	Mar. 29, 2025 EUR (€)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Settlement of net investment hedges \| $		$ 22	$ 0
Impairment charges \| $				$ 172
Fair value of long-term indebtedness greater/(less) than book value \| $		$ 77
Cross-currency swaps
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Settlement of net investment hedges \| €	€ 22
Cross-Currency Swap Maturing November 2027
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative notional amount \| €					€ 250
Cross-Currency Swap Maturing November 2029
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative notional amount \| €					€ 425