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Investments - Company's Investment in ABCP (Detail) (ABCP [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
ABCP [Member] |  |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |  |
Balance, beginning of year | $ 92 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | $ 90 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | $ 82 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember
Valuation adjustment | 3 mga_ValuationAdjustmentImpairment / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | 8 mga_ValuationAdjustmentImpairment / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | 15 mga_ValuationAdjustmentImpairment / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember
Cash receipts |  |  | (9)mga_CashReceipts / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember
Foreign exchange and other | (7)mga_ForeignExchangeAndOther / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | (6)mga_ForeignExchangeAndOther / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | 2 mga_ForeignExchangeAndOther / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember
Balance, end of year | $ 88 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | $ 92 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember | $ 90 mga_InvestmentCarryingValue / us-gaap_GainLossOnInvestmentsByCategoryAxis = mga_AbcpMember

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- Definition Cash receipts. + References No definition available. + Details
Name: | mga_CashReceipts
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Foreign exchange and other. + References No definition available. + Details
Name: | mga_ForeignExchangeAndOther
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Investment Carrying Value. + References No definition available. + Details
Name: | mga_InvestmentCarryingValue
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Valuation adjustment impairment. + References No definition available. + Details
Name: | mga_ValuationAdjustmentImpairment
Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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