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Employee Equity and Profit Participation Program - Additional Information (Detail) (Trust for Benefit of Employees [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Trust for Benefit of Employees [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Amount borrowed from company to purchase common shares maximum | $ 63 mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember | $ 39 mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember | $ 18 mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember
Amount included in accounts receivable | $ 63 mga_TrustIndebtednessReceivable / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember | $ 39 mga_TrustIndebtednessReceivable / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember |

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

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- Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration