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Employee Equity and Profit Participation Program - Additional Information (Detail) - Trust for Benefit of Employees [Member] - USD ($) | Dec. 31, 2017 | Dec. 31, 2016
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Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Amount borrowed from company to purchase common shares maximum | $ 19,000,000 | $ 10,000,000
Indebtedness receivable | $ 19,000,000 | $ 10,000,000

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- Definition Amount borrowed from company to purchase common shares maximum. + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Trust Indebtedness Receivable + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |