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v2.4.1.9

Income Taxes - Summary of Income Tax Rate (Detail) | 12 Months Ended
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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

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