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Financial Instruments - Additional Information (Detail)
In Millions, unless otherwise specified
12 Months Ended 12 Months Ended
Dec. 31, 2014
USD ($)
Dec. 31, 2013
USD ($)
Dec. 31, 2014
Fair Value, Inputs, Level 1 [Member]
Senior Notes [Member]
USD ($)
Dec. 31, 2014
Asset-backed Securities [Member]
CAD
Dec. 31, 2013
Asset-backed Securities [Member]
CAD
Dec. 31, 2014
Sales Revenue, Net [Member]
Customer
Dec. 31, 2014
Sales Revenue, Net [Member]
Customer Concentration Risk [Member]
Dec. 31, 2014
Forward Contracts [Member]
USD ($)
Derivative Instruments, Gain (Loss) [Line Items]                
Foreign exchange forward contracts, gain recognized in other comprehensive income               $ 30us-gaap_DerivativeInstrumentsGainRecognizedInOtherComprehensiveIncomeEffectivePortion
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForwardContractsMember
Foreign exchange forward contracts, loss recognized in other comprehensive income               174us-gaap_DerivativeInstrumentsLossRecognizedInOtherComprehensiveIncomeEffectivePortion
/ us-gaap_DerivativeByNatureAxis
= us-gaap_ForwardContractsMember
Face value of investments       107us-gaap_TradingSecuritiesCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
107us-gaap_TradingSecuritiesCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
     
Trading Securities 1,341us-gaap_TradingSecurities 1,646us-gaap_TradingSecurities   102us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
99us-gaap_TradingSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
     
Fair value of investments carrying value 5us-gaap_AvailableForSaleSecuritiesAmortizedCost 4us-gaap_AvailableForSaleSecuritiesAmortizedCost            
Long-term debt due within one year 184us-gaap_LongTermDebtCurrent 230us-gaap_LongTermDebtCurrent            
Debt instrument maturity period 1 year              
Estimated fair value of notes payable     $ 755us-gaap_NotesPayableFairValueDisclosure
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
         
Percentage of sales to six largest customers             83.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CustomerConcentrationRiskMember
 
Number of largest customers           6mga_NumberOfLargestCustomers
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= us-gaap_SalesRevenueNetMember