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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
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	$ 63mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember	$ 39mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember	$ 18mga_AmountBorrowedFromCompanyToPurchaseCommonSharesMaximum / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember
Amount included in accounts receivable	$ 63mga_TrustIndebtednessReceivable / us-gaap_StatementScenarioAxis = us-gaap_TrustForBenefitOfEmployeesMember	$ 39mga_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

X

- 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