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Long-Term Employee Benefit Liabilities - Company's Termination and Long Service Arrangements (Detail) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]      
Long-term employee benefit liabilities $ 580us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent $ 532us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent $ 560us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
Termination and Long Service Arrangements [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Beginning of year 354us-gaap_DefinedBenefitPlanBenefitObligation
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= mga_TerminationAndLongServiceArrangementsMember
314us-gaap_DefinedBenefitPlanBenefitObligation
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= mga_TerminationAndLongServiceArrangementsMember
252us-gaap_DefinedBenefitPlanBenefitObligation
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= mga_TerminationAndLongServiceArrangementsMember
Current service cost 20us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
24us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
16us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
Interest cost 12us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
13us-gaap_DefinedBenefitPlanInterestCost
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= mga_TerminationAndLongServiceArrangementsMember
13us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Actuarial losses and changes in actuarial assumptions 15us-gaap_DefinedBenefitPlanActuarialGainLoss
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12us-gaap_DefinedBenefitPlanActuarialGainLoss
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= mga_TerminationAndLongServiceArrangementsMember
41us-gaap_DefinedBenefitPlanActuarialGainLoss
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= mga_TerminationAndLongServiceArrangementsMember
Benefits paid (18)mga_DefinedBenefitPlanProjectedBenefitObligationBenefitsPaid
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
(21)mga_DefinedBenefitPlanProjectedBenefitObligationBenefitsPaid
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
(13)mga_DefinedBenefitPlanProjectedBenefitObligationBenefitsPaid
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Acquisition     2us-gaap_DefinedBenefitPlanBusinessCombinationsAndAcquisitionsBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Curtailment     (4)us-gaap_DefinedBenefitPlanCurtailments
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Foreign exchange (43)us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
12us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
7us-gaap_DefinedBenefitPlanForeignCurrencyExchangeRateChangesBenefitObligation
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= mga_TerminationAndLongServiceArrangementsMember
Ending funded status 340us-gaap_DefinedBenefitPlanFundedStatusOfPlan
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354us-gaap_DefinedBenefitPlanFundedStatusOfPlan
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314us-gaap_DefinedBenefitPlanFundedStatusOfPlan
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Current liability 10us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
11us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities
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= mga_TerminationAndLongServiceArrangementsMember
10us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansCurrentLiabilities
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Long-term employee benefit liabilities 330us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
343us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
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= mga_TerminationAndLongServiceArrangementsMember
304us-gaap_PensionAndOtherPostretirementDefinedBenefitPlansLiabilitiesNoncurrent
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Net amount 340us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet
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= mga_TerminationAndLongServiceArrangementsMember
354us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
314us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet
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= mga_TerminationAndLongServiceArrangementsMember
Unrecognized actuarial losses (81)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
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(82)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized
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= mga_TerminationAndLongServiceArrangementsMember
(74)us-gaap_DefinedBenefitPlanBeforeAdoptionOfSFAS158RecognitionProvisionsNetGainsLossesNotYetRecognized
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= mga_TerminationAndLongServiceArrangementsMember
Current service cost 20us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
24us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
16us-gaap_DefinedBenefitPlanServiceCost
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= mga_TerminationAndLongServiceArrangementsMember
Interest cost 12us-gaap_DefinedBenefitPlanInterestCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
13us-gaap_DefinedBenefitPlanInterestCost
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= mga_TerminationAndLongServiceArrangementsMember
13us-gaap_DefinedBenefitPlanInterestCost
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= mga_TerminationAndLongServiceArrangementsMember
Actuarial losses 16us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
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= mga_TerminationAndLongServiceArrangementsMember
4us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
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12us-gaap_DefinedBenefitPlanAmortizationOfGainsLosses
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
Net periodic benefit cost $ 48us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
$ 41us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember
$ 41us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mga_TerminationAndLongServiceArrangementsMember