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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 |  |  |  |  |  |  |  | $ 30 us-gaap_DerivativeInstrumentsGainRecognizedInOtherComprehensiveIncomeEffectivePortion / us-gaap_DerivativeByNatureAxis = us-gaap_ForwardContractsMember
Foreign exchange forward contracts, loss recognized in other comprehensive income |  |  |  |  |  |  |  | 174 us-gaap_DerivativeInstrumentsLossRecognizedInOtherComprehensiveIncomeEffectivePortion / us-gaap_DerivativeByNatureAxis = us-gaap_ForwardContractsMember
Face value of investments |  |  |  | 107 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 107 us-gaap_TradingSecuritiesCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |  |
Trading Securities | 1,341 us-gaap_TradingSecurities | 1,646 us-gaap_TradingSecurities |  | 102 us-gaap_TradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 99 us-gaap_TradingSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |  |  |
Fair value of investments carrying value | 5 us-gaap_AvailableForSaleSecuritiesAmortizedCost | 4 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  |  |  |  |  |
Long-term debt due within one year | 184 us-gaap_LongTermDebtCurrent | 230 us-gaap_LongTermDebtCurrent |  |  |  |  |  |
Debt instrument maturity period | 1 year |  |  |  |  |  |  |
Estimated fair value of notes payable |  |  | $ 755 us-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 |  |  |  |  |  | 6 mga_NumberOfLargestCustomers / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember |  |

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- Definition Maturity period of term loan facility under sub tranche. + References No definition available. + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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- Definition The effective portion of gains on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition The effective portion of losses on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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- Definition Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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