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Income Taxes - Summary of Income Tax Rate (Detail)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Canadian statutory income tax rate 26.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 26.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 26.50%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Manufacturing and processing profits deduction (0.40%)mga_EffectiveIncomeTaxRateReconciliationDeductionsManufacturingAndProcessingProfits (0.40%)mga_EffectiveIncomeTaxRateReconciliationDeductionsManufacturingAndProcessingProfits (0.70%)mga_EffectiveIncomeTaxRateReconciliationDeductionsManufacturingAndProcessingProfits
Foreign rate differentials (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential (1.50%)us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential
Losses not benefited 1.30%mga_EffectiveIncomeTaxRateReconciliationLossesNotBenefited 5.30%mga_EffectiveIncomeTaxRateReconciliationLossesNotBenefited 5.80%mga_EffectiveIncomeTaxRateReconciliationLossesNotBenefited
Utilization of losses previously not benefited (0.30%)mga_UtilizationOfLossesPreviouslyNotBenefited (1.00%)mga_UtilizationOfLossesPreviouslyNotBenefited (0.30%)mga_UtilizationOfLossesPreviouslyNotBenefited
Earnings of equity accounted investees (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary (1.20%)us-gaap_EffectiveIncomeTaxRateReconciliationEquityInEarningsLossesOfUnconsolidatedSubsidiary
Tax on repatriation of foreign earnings 0.60%us-gaap_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarnings 1.40%us-gaap_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarnings  
Valuation allowance on deferred tax assets (0.10%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (1.10%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance (5.00%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Austrian tax reform 1.30%us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingenciesForeign    
Mexican flat tax   (1.90%)us-gaap_EffectiveIncomeTaxRateReconciliationChangeInEnactedTaxRate   
Research and development tax credits (1.60%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch (4.30%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch (2.30%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch
Reserve for uncertain tax positions (1.70%)mga_EffectiveIncomeTaxRateReconciliationIncomeTaxReserveForUncertainTaxPosition (2.30%)mga_EffectiveIncomeTaxRateReconciliationIncomeTaxReserveForUncertainTaxPosition (1.00%)mga_EffectiveIncomeTaxRateReconciliationIncomeTaxReserveForUncertainTaxPosition
Re-measurement gains     (1.10%)mga_EffectiveIncomeTaxRateReconciliationRemeasurementGain
Others 1.60%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.70%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments (0.70%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective income tax rate 26.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations 18.90%us-gaap_EffectiveIncomeTaxRateContinuingOperations 18.50%us-gaap_EffectiveIncomeTaxRateContinuingOperations