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Investments - Company's Investment in ABCP (Detail) (ABCP [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
ABCP [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Balance, beginning of year | $ 82 | $ 84 | $ 85
Valuation adjustment | 15 |  |
Cash receipts | (9) |  | (5)
Foreign exchange and other | 2 | (2) | 4
Balance, end of year | $ 90 | $ 82 | $ 84

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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

X
- 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_ScheduleOfEquityMethodInvestmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration