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Long-Term Employee Benefit Liabilities - Schedule of Company's Defined Benefit Pension Plans (Parenthetical) (Detail) (Pension Plan, Defined Benefit [Member]) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |
Equity securities | 58.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations | 58.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations |
Fixed income securities | 41.00%mga_DefinedBenefitPlanTargetAllocationPercentageOfAssetsDebtFixedIncomeSecurities | 41.00%mga_DefinedBenefitPlanTargetAllocationPercentageOfAssetsDebtFixedIncomeSecurities |
Cash and cash equivalents | 1.00%mga_DefinedBenefitPlanCashAndCashEquivalents | 1.00%mga_DefinedBenefitPlanCashAndCashEquivalents |
Total | 100.00%mga_DefinedBenefitPlansTargetAllocationPercentage | 100.00%mga_DefinedBenefitPlansTargetAllocationPercentage |
Subsequent Event [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total |  |  | 100.00%mga_DefinedBenefitPlansTargetAllocationPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Minimum [Member] | Subsequent Event [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Equity securities |  |  | 55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Fixed income securities |  |  | 25.00%mga_DefinedBenefitPlanTargetAllocationPercentageOfAssetsDebtFixedIncomeSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Cash and cash equivalents |  |  | 0.00%mga_DefinedBenefitPlanCashAndCashEquivalents / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Maximum [Member] | Subsequent Event [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Equity securities |  |  | 75.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Fixed income securities |  |  | 45.00%mga_DefinedBenefitPlanTargetAllocationPercentageOfAssetsDebtFixedIncomeSecurities / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Cash and cash equivalents |  |  | 15.00%mga_DefinedBenefitPlanCashAndCashEquivalents / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- Definition Defined benefit plan cash and cash equivalents. + References No definition available. + Details
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- Definition Defined benefit plans target allocation percentage. + References No definition available. + Details
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- Definition Defined Benefit Plan Target Allocation Percentage Of Assets Debt Fixed Income Securities + References No definition available. + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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