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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
	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

X

- Definition

Defined benefit plan cash and cash equivalents.

+ References

No definition available.

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- Definition

Defined benefit plans target allocation percentage.

+ References

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- Definition

Defined Benefit Plan Target Allocation Percentage Of Assets Debt Fixed Income Securities

+ References

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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

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