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Other Expense (Income), Net - Other Expense (Income), Net (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended | 1 Months Ended | 12 Months Ended | 0 Months Ended | 3 Months Ended
--- | --- | --- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Aug. 31, 2012 | Dec. 31, 2014 | Oct. 26, 2012 | Dec. 31, 2012
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Impairment of goodwill | $ 22 us-gaap_GoodwillImpairmentLoss |  |  |  |  |
STT Technologies Inc. [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Company acquired |  |  |  |  | 50.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember |
E-Car [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Company acquired |  |  | 27.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_ECarMember |  |  |
Corporate [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Estimated fair value |  | 205 mga_EstimatedCostFairValueInput / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |  |
Non-cash gain |  | 153 us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |  |
Non-cash gain after tax |  | 125 mga_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGainAfterTax / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |  |
Corporate [Member] | E-Car [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Company acquired |  |  | 27.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_ECarMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |
Cash acquired consideration |  |  | 75 us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates / us-gaap_BusinessAcquisitionAxis = mga_ECarMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |
Percentage of non-controlling interest in an equity accounted investment |  |  | 73.00%mga_PercentageOfNoncontrollingInterestInEquityAccountedInvestment / us-gaap_BusinessAcquisitionAxis = mga_ECarMember / us-gaap_StatementBusinessSegmentsAxis = us-gaap_CorporateMember |  |  |
North America [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Long-lived asset impairment charges | 23 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 2 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  | 18 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |
Long-lived asset impairment charges, after tax | 11 mga_LongLivedAssetsImpairmentChargesAfterTax / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember | 1 mga_LongLivedAssetsImpairmentChargesAfterTax / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  | 12 mga_LongLivedAssetsImpairmentChargesAfterTax / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |
Non-cash gain |  | 35 us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |  |  |
North America [Member] | STT Technologies Inc. [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Company acquired |  |  |  |  | 50.00%mga_BusinessAcquisitionPercentageOfInterestAcquired / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |
Cash consideration |  |  |  |  | 55 us-gaap_PaymentsToAcquireBusinessesGross / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |
Estimated fair value |  | 55 mga_EstimatedCostFairValueInput / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |  |  |
Non-cash gain |  | 35 us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |  |  |
Non-cash gain after tax |  | 35 mga_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGainAfterTax / us-gaap_BusinessAcquisitionAxis = mga_SttTechnologiesMember / us-gaap_StatementGeographicalAxis = us-gaap_NorthAmericaMember |  |  |  |
Europe [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Long-lived asset impairment charges |  | 23 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |  |  |  | 23 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember
Long-lived asset impairment charges, after tax |  |  |  |  |  | 22 mga_LongLivedAssetsImpairmentChargesAfterTax / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember
Restructuring charges | 89 us-gaap_RestructuringCharges / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 55 us-gaap_RestructuringCharges / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |  | 46 us-gaap_RestructuringCharges / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |  |
Restructuring charges, after tax | 64 mga_RestructuringChargesNetOfTax / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember | 53 mga_RestructuringChargesNetOfTax / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |  | 41 mga_RestructuringChargesNetOfTax / us-gaap_StatementGeographicalAxis = us-gaap_EuropeMember |  |
Rest of World [Member] |  |  |  |  |  |
Component of Operating Other Cost and Expense [Abstract] |  |  |  |  |  |
Long-lived asset impairment charges | 10 us-gaap_ImpairmentOfLongLivedAssetsHeldForUse / us-gaap_StatementGeographicalAxis = mga_RestOfWorldMember |  |  |  |  |
Long-lived asset impairment charges, after tax | 10 mga_LongLivedAssetsImpairmentChargesAfterTax / us-gaap_StatementGeographicalAxis = mga_RestOfWorldMember |  |  |  |  |
Impairment of goodwill | 22 us-gaap_GoodwillImpairmentLoss / us-gaap_StatementGeographicalAxis = mga_RestOfWorldMember |  |  |  |  |
Impairment of goodwill, after tax | $ 22 us-gaap_GoodwillImpairmentLossNetOfTax / us-gaap_StatementGeographicalAxis = mga_RestOfWorldMember |  |  |  |  |

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