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Significant Accounting Policies - Additional Information (Detail)
 12 Months Ended
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