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Accumulated Other Comprehensive Income - Income Tax Benefit in Accumulated Net Unrealized Loss on Other Long-Term Liabilities (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Balance, beginning of year | $ 24 | $ 1 | $ 6
Net unrealized loss (gain) | 15 | 25 | (3)
Reclassification of net loss to net income | (3) | (2) | (2)
Balance, end of year | $ 36 | $ 24 | $ 1

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- Definition Accumulated other comprehensive income other long term liabilities net of tax. + References No definition available. + Details
Name: | mga_AccumulatedOtherComprehensiveIncomeOtherLongTermLiabilitiesNetOfTax
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Other Comprehensive Income (Loss) on Other Long Term Liabilities Reclassification Adjustment on Derivatives Included in Net Income, Tax + References No definition available. + Details
Name: | mga_OtherComprehensiveIncomeLossOnOtherLongTermLiabilitiesReclassificationAdjustmentOnDerivativesIncludedInNetIncomeTax
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Other comprehensive income net unrealized Gain loss on other long term liabilities net of tax. + References No definition available. + Details
Name: | mga_OtherComprehensiveIncomeNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetOfTax
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration