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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
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Balance, beginning of year	$ 4mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges	$ (13)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges	$ 12mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges
Net unrealized loss (gain)	41mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss	11mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss	(30)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesNetUnrealizedGainLoss
Reclassification of net (loss) gain to net income	(1)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome	6mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome	5mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedgesReclassificationOfNetGainToNetIncome
Balance, end of year	44mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges	4mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges	(13)mga_AccumulatedOtherComprehensiveIncomeLossIncomeTaxObligationNettedInAccumulatedNetUnrealizedGainLossOnCashFlowHedges
Balance, beginning of year	14mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities	36mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities	24mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities
Net unrealized loss (gain)	23mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss	(21)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss	15mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesNetUnrecognizedGainLoss
Reclassification of net loss to net income	(1)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome	(1)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome	(3)mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilitiesReclassificationOfNetGainLossToNetIncome
Balance, end of year	36mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities	14mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities	36mga_AccumulatedOtherComprehensiveIncomeIncomeTaxBenefitLossNettedInAccumulatedNetUnrealizedGainLossOnOtherLongTermLiabilities
Total income tax benefit	$ 80us-gaap_OtherComprehensiveIncomeLossTax	$ 18us-gaap_OtherComprehensiveIncomeLossTax	$ 23us-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

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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

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