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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
	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			1.10195mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember	1.47805mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember					1.07317mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember	1.48303mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember				1.09970mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember			0.87961mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember					0.07221mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember			0.07135mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember		0.06884mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember					0.76616mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember	1.19170mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember	0.03800mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember						0.76905mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember		0.03698mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember					0.78725mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember		0.03656mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_CzechKorunaMember
Foreign exchange forward contract, Sales Exchange Rate			0.92552mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember	0.69344mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_EuroMember					0.91253mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember					0.89597mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember					0.87043mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodNineMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_InvestmentTypeAxis = mga_USDollarMember			13.47236mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_InvestmentTypeAxis = mga_MexicanPesoMember										1.31120mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember	0.85706mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember							1.30607mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember	0.80758mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember						1.33328mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember	0.81324mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_BritishPoundMember						1.31030mga_ForeignCurrencyForwardSaleContractWeightedAverageRate / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodEightMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_InvestmentTypeAxis = mga_USDollarMember	0.81930mga_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	€ 50invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_CAD			$ 235invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember	€ 42invest_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			$ 30invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_CAD / us-gaap_PositionAxis = us-gaap_LongMember	€ 23invest_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		$ 2invest_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	7,944invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_USD		4,390invest_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		2,766invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_USD / us-gaap_PositionAxis = us-gaap_LongMember		911invest_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	4,935invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_EUR	£ (25)invest_DerivativeNotionalAmount / us-gaap_FinancialInstrumentAxis = currency_EUR				$ 85invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember	2,571invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodFiveMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember	£ 13invest_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				$ 24invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSixMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember	1,609invest_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				$ 3invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mga_ForeignExchangeForwardContractsPeriodSevenMember / us-gaap_FinancialInstrumentAxis = currency_EUR / us-gaap_PositionAxis = us-gaap_LongMember	755invest_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

X

- 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

X

- 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

X

- 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

X

- Details

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