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Accumulated Other Comprehensive Loss - Additional Information (Detail) $ in Millions | Dec. 31, 2020 USD ($)
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Equity [Abstract] |
Amount of comprehensive loss that is expected to be reclassified to net income | $ 44

X
- Definition Other comprehensive income that is expected to be reclassified to net income over the next fiscal year, net of tax. + References No definition available. + Details
Name: | mga_OtherComprehensiveIncomeToBeReclassifiedToNetIncome
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_EquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration