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

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Deferred Tax Asset -URI http://asc.fasb.org/extlink&oid=6510090 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Carryforwards -URI http://asc.fasb.org/extlink&oid=6506874 + Details
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