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Accumulated Other Comprehensive (Loss) Income - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014
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Equity [Abstract] |
Amount of other comprehensive income is expected to be reclassified to net income during 2014 | $ 55 mga_OtherComprehensiveIncomeToBeReclassifiedToNetIncome
Amount of other comprehensive income net of income tax during 2014 | $ 21 mga_IncomeTaxOfOtherComprehensiveIncomeToBeReclassifiedToNetIncome

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- Definition Income Tax portion of other comprehensive income that is expected to be reclassified to net income over the next fiscal year. + References No definition available. + Details
Name: | mga_IncomeTaxOfOtherComprehensiveIncomeToBeReclassifiedToNetIncome
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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