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Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Undistributed earnings of certain foreign subsidiaries	$ 3,990,000,000us-gaap_UndistributedEarningsOfForeignSubsidiaries
Income taxes paid, net	547,000,000us-gaap_IncomeTaxesPaidNet	507,000,000us-gaap_IncomeTaxesPaidNet	347,000,000us-gaap_IncomeTaxesPaidNet
Income tax loss carryforwards	2,270,000,000us-gaap_OperatingLossCarryforwards
Losses on acquisition	25,000,000us-gaap_TaxCreditCarryforwardAmount
Total losses	1,610,000,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsNotSubjectToExpiration
Expiry date of losses	Between 2015 and 2034
Gross unrecognized tax benefits	202,000,000us-gaap_UnrecognizedTaxBenefits	238,000,000us-gaap_UnrecognizedTaxBenefits	279,000,000us-gaap_UnrecognizedTaxBenefits	252,000,000us-gaap_UnrecognizedTaxBenefits
Recognized tax benefits	177,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	219,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate	240,000,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate
Interest and penalties on the unrecognized tax benefits	24,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	42,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued	49,000,000us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued
Recoveries/(expenses) related to changes in reserves for interest and penalties	18,000,000mga_UnrecognizedTaxBenefitsIncomeTaxRecoveryRelatedToChangesInPenaltiesAndInterest	7,000,000mga_UnrecognizedTaxBenefitsIncomeTaxRecoveryRelatedToChangesInPenaltiesAndInterest	(7,000,000)mga_UnrecognizedTaxBenefitsIncomeTaxRecoveryRelatedToChangesInPenaltiesAndInterest
Decrease in gross unrecognized tax benefits	38,000,000us-gaap_SignificantChangeInUnrecognizedTaxBenefitsIsReasonablyPossibleAmountOfUnrecordedBenefit
Decrease in recognized tax benefits	$ 35,000,000mga_SignificantChangeInUnrecognizedTaxBenefitsReasonablyPossibleAmountOfUnrecordedBenefitThatWouldImpactEffectiveTaxRate

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The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes.

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Amount of undistributed earnings of foreign subsidiaries intended to be permanently reinvested outside the country of domicile.

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