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Business Acquisitions - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012	Nov. 30, 2013	Oct. 31, 2014	Oct. 26, 2012
Business Acquisition [Line Items]
Increase in fixed assets	$ 21,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment	$ 5,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
Increase in goodwill	3,000,000us-gaap_GoodwillPeriodIncreaseDecrease	3,000,000us-gaap_GoodwillPeriodIncreaseDecrease
Increase in other assets	4,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets	2,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
Long-term debt	4,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt
Deferred tax liabilities	1,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent
Other long-term liabilities		2,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther
Reduction of non-controlling interest		1,000,000us-gaap_MinorityInterestDecreaseFromRedemptions
Net cash outflow	23,000,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired	9,000,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired	525,000,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired
Company's pro forma consolidated sales	36,700,000,000us-gaap_BusinessAcquisitionsProFormaRevenue	34,900,000,000us-gaap_BusinessAcquisitionsProFormaRevenue
Proforma consolidated income	1,900,000,000us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss	1,600,000,000us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss
E-Car [Member]
Business Acquisition [Line Items]
Interest acquired				27.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_ECarMember
Net cash outflow				56,000,000us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired / us-gaap_BusinessAcquisitionAxis = mga_ECarMember
Cash acquired				19,000,000us-gaap_CashAcquiredFromAcquisition / us-gaap_BusinessAcquisitionAxis = mga_ECarMember
Textile Competence Centre Kft [Member]
Business Acquisition [Line Items]
Cash paid					9,000,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = mga_TextileCompetenceCentreKftMember
Interest acquired					49.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_TextileCompetenceCentreKftMember
STT Technologies Inc. [Member]
Business Acquisition [Line Items]
Interest acquired							50.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember
Techform Group Of Companies [Member]
Business Acquisition [Line Items]
Cash paid						$ 23,000,000us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = mga_TechformGroupOfCompaniesMember
Acquisition date						2014-10

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