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Accumulated Other Comprehensive (Loss) Income - Summary of Income Tax Benefit to Component of Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Equity [Abstract] |  |  |
Balance, beginning of year | $ 4 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges | $ (13)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges | $ 12 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges
Net unrealized loss (gain) | 41 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss | 11 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss | (30)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss
Reclassification of net (loss) gain to net income | (1)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome | 6 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome | 5 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome
Balance, end of year | 44 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges | 4 mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges | (13)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges
Balance, beginning of year | 14 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities | 36 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities | 24 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities
Net unrealized loss (gain) | 23 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss | (21)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss | 15 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss
Reclassification of net loss to net income | (1)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome | (1)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome | (3)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome
Balance, end of year | 36 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities | 14 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities | 36 mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities
Total income tax benefit | $ 80 us-gaap_OtherComprehensiveIncomeLossTax | $ 18 us-gaap_OtherComprehensiveIncomeLossTax | $ 23 us-gaap_OtherComprehensiveIncomeLossTax

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- Definition Accumulated Other Comprehensive Income, Income Tax Benefit Loss Netted In Accumulated Net Unrealized Gain Loss On Other Long Term Liabilities + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income, Income Tax Benefit Loss Netted In Accumulated Net Unrealized Gain Loss On Other Long Term Liabilities Net Unrecognized Gain Loss + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income, Income Tax Benefit Loss Netted In Accumulated Net Unrealized Gain Loss On Other Long Term Liabilities Reclassification Of Net Gain Loss To Net Income + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss, Income Tax Netted In Accumulated Net Unrealized Gain Loss On Cash Flow Hedges Net Unrealized Gain Loss + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss Income Tax Netted In Accumulated Net Unrealized Gain Loss On Cash Flow Hedges Reclassification Of Net Gain To Net Income + References No definition available. + Details
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- Definition Accumulated Other Comprehensive Income Loss Income Tax Obligation Netted In Accumulated Net Unrealized Gain Loss On Cash Flow Hedges + References No definition available. + Details
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- Definition Amount of tax expense (benefit) allocated to other comprehensive income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 20 -Section 45 -Paragraph 11 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=18498875&loc=d3e39076-109324 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580 + Details
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