Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Tax benefit of loss carryforwards	$ 712	$ 610
Liabilities currently not deductible for tax	238mga_DeferredTaxAssetsOtherLiabilities	342mga_DeferredTaxAssetsOtherLiabilities
Tax credit carryforwards	25	34
Unrealized loss on cash flow hedges and retirement liabilities	120	39
Others	11us-gaap_DeferredTaxAssetsOther	11us-gaap_DeferredTaxAssetsOther
Deferred tax assets, Gross, Total	1,106us-gaap_DeferredTaxAssetsGross	1,036us-gaap_DeferredTaxAssetsGross
Valuation allowance against tax benefit of loss carryforwards	(663)	(528)
Other valuation allowance	(79)	(111)
Deferred tax assets, Net, Total	364us-gaap_DeferredTaxAssetsNet	397us-gaap_DeferredTaxAssetsNet
Tax depreciation in excess of book depreciation	208	170
Other assets book value in excess of tax value		15
Tax on undistributed foreign earnings	7	5
Unrealized gain on cash flow hedges and retirement liabilities	9	21
Deferred tax liabilities, Total	224us-gaap_DeferredTaxLiabilities	211us-gaap_DeferredTaxLiabilities
Net deferred tax assets	$ 140us-gaap_DeferredTaxAssetsLiabilitiesNet	$ 186us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred tax assets other liabilities.

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Deferred tax assets valuation allowance other.

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Deferred tax liabilities other assets book value in excess of tax value.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from hedging transactions.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards that are subject to expiration dates.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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The tax effect as of the balance sheet date of the amount of future tax deductions arising from all unused tax credit carryforwards which have been reduced by a valuation allowance.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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Amount of deferred tax liability attributable to taxable temporary differences from derivatives.

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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Amount of deferred tax liability attributable to taxable temporary differences from undistributed earnings of subsidiaries and other recognized entities not within the country of domicile of the entity.

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