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Accumulated Other Comprehensive Loss - Additional Information (Detail) $ in Millions | Dec. 31, 2018 USD ($)
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Scenario, Forecast [Member] |
Accumulated Other Comprehensive Income (Loss) [Line Items] |
Amount of comprehensive loss that is expected to be reclassified to net income | $ 21

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- 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
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Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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_AccumulatedOtherComprehensiveIncomeLossLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |