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Financial Instruments - Foreign Exchange Forward Contracts Representing Commitments to Buy and Sell Various Foreign Currencies (Detail) In Millions, unless otherwise specified |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |
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Dec. 31, 2014 Canadian Dollar [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] EUR (€) | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] U S Dollar [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] Euro [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] Long [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] Long [Member] EUR (€) | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] Short [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2015 [Member] Short [Member] EUR (€) | Dec. 31, 2014 Canadian Dollar [Member] 2016 [Member] U S Dollar [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2016 [Member] Euro [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2016 [Member] Long [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2016 [Member] Long [Member] EUR (€) | Dec. 31, 2014 Canadian Dollar [Member] 2016 [Member] Short [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2017 [Member] U S Dollar [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2017 [Member] Long [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2017 [Member] Short [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2018 [Member] U S Dollar [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2018 [Member] Long [Member] USD ($) | Dec. 31, 2014 Canadian Dollar [Member] 2019 [Member] U S Dollar [Member] | Dec. 31, 2014 Canadian Dollar [Member] 2019 [Member] Short [Member] USD ($) | Dec. 31, 2014 U S Dollar [Member] MXN | Dec. 31, 2014 U S Dollar [Member] 2015 [Member] Mexican Peso [Member] | Dec. 31, 2014 U S Dollar [Member] 2015 [Member] Long [Member] MXN | Dec. 31, 2014 U S Dollar [Member] 2015 [Member] Short [Member] MXN | Dec. 31, 2014 U S Dollar [Member] 2016 [Member] Mexican Peso [Member] | Dec. 31, 2014 U S Dollar [Member] 2016 [Member] Long [Member] MXN | Dec. 31, 2014 U S Dollar [Member] 2017 [Member] Mexican Peso [Member] | Dec. 31, 2014 U S Dollar [Member] 2017 [Member] Long [Member] MXN | Dec. 31, 2014 Euros [Member] USD ($) | Dec. 31, 2014 Euros [Member] CZK | Dec. 31, 2014 Euros [Member] GBP (£) | Dec. 31, 2014 Euros [Member] 2015 [Member] U S Dollar [Member] | Dec. 31, 2014 Euros [Member] 2015 [Member] British Pound [Member] | Dec. 31, 2014 Euros [Member] 2015 [Member] Czech Koruna [Member] | Dec. 31, 2014 Euros [Member] 2015 [Member] Long [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2015 [Member] Long [Member] CZK | Dec. 31, 2014 Euros [Member] 2015 [Member] Long [Member] GBP (£) | Dec. 31, 2014 Euros [Member] 2015 [Member] Short [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2015 [Member] Short [Member] GBP (£) | Dec. 31, 2014 Euros [Member] 2016 [Member] U S Dollar [Member] | Dec. 31, 2014 Euros [Member] 2016 [Member] British Pound [Member] | Dec. 31, 2014 Euros [Member] 2016 [Member] Czech Koruna [Member] | Dec. 31, 2014 Euros [Member] 2016 [Member] Long [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2016 [Member] Long [Member] CZK | Dec. 31, 2014 Euros [Member] 2016 [Member] Short [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2016 [Member] Short [Member] GBP (£) | Dec. 31, 2014 Euros [Member] 2017 [Member] U S Dollar [Member] | Dec. 31, 2014 Euros [Member] 2017 [Member] British Pound [Member] | Dec. 31, 2014 Euros [Member] 2017 [Member] Czech Koruna [Member] | Dec. 31, 2014 Euros [Member] 2017 [Member] Long [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2017 [Member] Long [Member] CZK | Dec. 31, 2014 Euros [Member] 2017 [Member] Short [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2017 [Member] Short [Member] GBP (£) | Dec. 31, 2014 Euros [Member] 2018 [Member] U S Dollar [Member] | Dec. 31, 2014 Euros [Member] 2018 [Member] British Pound [Member] | Dec. 31, 2014 Euros [Member] 2018 [Member] Short [Member] USD ($) | Dec. 31, 2014 Euros [Member] 2018 [Member] Short [Member] GBP (£)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign exchange forward contract, Purchases Exchange Rate |  |  | 1.10195 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 1.47805 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember |  |  |  |  | 1.07317 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 1.48303 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember |  |  |  | 1.09970 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  |  | 0.87961 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  |  |  |  | 0.07221 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember |  |  | 0.07135 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember |  | 0.06884 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember |  |  |  |  | 0.76616 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 1.19170 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember | 0.03800 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember |  |  |  |  |  | 0.76905 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  | 0.03698 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember |  |  |  |  | 0.78725 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  | 0.03656 mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember |  |  |  |  |  |  |  |
Foreign exchange forward contract, Sales Exchange Rate |  |  | 0.92552 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 0.69344 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember |  |  |  |  | 0.91253 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  |  |  |  | 0.89597 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  |  |  |  | 0.87043 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodNineMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember |  |  | 13.47236 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember |  |  |  |  |  |  |  |  |  | 1.31120 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 0.85706 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember |  |  |  |  |  |  | 1.30607 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 0.80758 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember |  |  |  |  |  | 1.33328 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 0.81324 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember |  |  |  |  |  | 1.31030 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember | 0.81930 mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember |  |
Notional amount of foreign currency derivatives | $ (1,061)invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_CAD | € 50 invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_CAD |  |  | $ 235 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember | € 42 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember | $ (617)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_ShortMember | € (15)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_ShortMember |  |  | $ 30 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember | € 23 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember | $ (370)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_ShortMember |  | $ 2 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember | $ (226)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_ShortMember |  | $ (76)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember |  | $ (39)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodNineMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_ShortMember | 7,944 invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_USD |  | 4,390 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_PositionAxis = us-gaap_LongMember | (123)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_PositionAxis = us-gaap_ShortMember |  | 2,766 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_PositionAxis = us-gaap_LongMember |  | 911 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_PositionAxis = us-gaap_LongMember | $ (176)invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_EUR | 4,935 invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_EUR | £ (25)invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_EUR |  |  |  | $ 85 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | 2,571 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | £ 13 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | $ (152)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember | £ (15)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  | $ 24 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | 1,609 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | $ (81)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember | £ (13)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember |  |  |  | $ 3 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | 755 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember | $ (43)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember | £ (9)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember |  |  | $ (12)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember | £ (1)invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_ShortMember

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Foreign currency forward purchase contract weighted average rate. + References No definition available. + Details
Name: | mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate
Namespace Prefix: | mga_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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- Definition Foreign currency forward sale contract weighted average rate. + References No definition available. + Details
Name: | mga_ForeignCurrencyForwardSaleContractWeightedAverageRate
Namespace Prefix: | mga_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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