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v2.4.1.9

Significant Accounting Policies - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2014
Significant Accounting Policies [Line Items] |
Short-term investments maturity period | Less than three months
Estimated useful lives of definite-lived intangible assets | 5 years
Changes in assumptions and experience gains and losses | Greater than 10%
Period of benefit obligation in current liabilities | Next twelve months
Minimum [Member] | Building [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 2.50%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | General Purpose Equipment [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 7.00%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mga_GeneralPurposeEquipmentMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | Special Purpose Equipment [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 10.00%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mga_SpecialPurposeEquipmentMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member] | Building [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 5.00%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_BuildingMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Maximum [Member] | General Purpose Equipment [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 10.00%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mga_GeneralPurposeEquipmentMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Maximum [Member] | Special Purpose Equipment [Member] |
Significant Accounting Policies [Line Items] |
Annual rates of depreciation on fixed assets | 33.00%mga_PropertyPlantAndEquipmentDepreciationRate / us-gaap_PropertyPlantAndEquipmentByTypeAxis = mga_SpecialPurposeEquipmentMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition Changes in assumptions and experience gains and losses recognized in income. + References No definition available. + Details
Name: | mga_ChangesInAssumptionsAndExperienceGainsAndLossesRecognizedInIncome
Namespace Prefix: | mga_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Period of benefit obligation in current liabilities. + References No definition available. + Details
Name: | mga_PeriodOfBenefitObligationInCurrentLiabilities
Namespace Prefix: | mga_
Data Type: | us-types:durationStringItemType
Balance Type: | na
Period Type: | duration

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- Definition Property plant and equipment depreciation rate. + References No definition available. + Details
Name: | mga_PropertyPlantAndEquipmentDepreciationRate
Namespace Prefix: | mga_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Short term investments maturity period. + References No definition available. + Details
Name: | mga_ShortTermInvestmentsMaturityPeriod
Namespace Prefix: | mga_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | mga_SignificantAccountingPoliciesLineItems
Namespace Prefix: | mga_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |