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Investments - Company's Investment in ABCP (Detail) (ABCP [Member], USD $)
In Millions, unless otherwise specified
 12 Months Ended
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 $ 92mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 $ 90mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 $ 82mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember
Valuation adjustment 3mga_ValuationAdjustmentImpairment
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 8mga_ValuationAdjustmentImpairment
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 15mga_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		 2mga_ForeignExchangeAndOther
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember
Balance, end of year $ 88mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 $ 92mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember		 $ 90mga_InvestmentCarryingValue
/ us-gaap_GainLossOnInvestmentsByCategoryAxis
= mga_AbcpMember