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Income Taxes - Summary of Deferred Income Taxes Provided on Temporary Differences (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax depreciation greater (less) than book depreciation	$ 41us-gaap_IncomeTaxReconciliationNondeductibleExpenseDepreciation	$ (23)us-gaap_IncomeTaxReconciliationNondeductibleExpenseDepreciation	$ 13us-gaap_IncomeTaxReconciliationNondeductibleExpenseDepreciation
Book amortization (in excess of) less than tax amortization	(24)us-gaap_IncomeTaxReconciliationNondeductibleExpenseAmortization	(57)us-gaap_IncomeTaxReconciliationNondeductibleExpenseAmortization	16us-gaap_IncomeTaxReconciliationNondeductibleExpenseAmortization
Liabilities currently not deductible for tax	18mga_IncomeTaxReconciliationLiabilitiesCurrentlyNotDeductibleForTax	(48)mga_IncomeTaxReconciliationLiabilitiesCurrentlyNotDeductibleForTax	(29)mga_IncomeTaxReconciliationLiabilitiesCurrentlyNotDeductibleForTax
Net tax losses utilized (benefited)	27mga_IncomeTaxReconciliationNetTaxLossesUtilized	50mga_IncomeTaxReconciliationNetTaxLossesUtilized	(11)mga_IncomeTaxReconciliationNetTaxLossesUtilized
Change in valuation allowance on deferred tax assets	(3)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	(21)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	(89)us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Austrian tax reform	32us-gaap_IncomeTaxReconciliationTaxContingenciesForeign
Net tax credits utilized	10us-gaap_IncomeTaxReconciliationTaxCredits	2us-gaap_IncomeTaxReconciliationTaxCredits	53us-gaap_IncomeTaxReconciliationTaxCredits
Others	(7)us-gaap_IncomeTaxReconciliationOtherAdjustments	(3)us-gaap_IncomeTaxReconciliationOtherAdjustments	1us-gaap_IncomeTaxReconciliationOtherAdjustments
Deferred income tax, Total	$ 94us-gaap_DeferredIncomeTaxExpenseBenefit	$ (100)us-gaap_DeferredIncomeTaxExpenseBenefit	$ (46)us-gaap_DeferredIncomeTaxExpenseBenefit

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Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to nondeductible amortization.

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