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Long-Term Employee Benefit Liabilities - Summary of Weighted Average Significant Actuarial Assumptions Adopted in Measuring Company's Projected Termination and Long Service Benefit Obligations and Net Periodic Benefit Cost (Detail) (Termination And Long Service Arrangements [Member]) | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
--- | --- | --- | ---
Termination And Long Service Arrangements [Member] |  |  |
Pension Plans, Postretirement and Other Employee Benefits [Line Items] |  |  |
Discount rate | 4.20% | 5.40% | 4.90%
Rate of compensation increase | 3.90% | 4.00% | 3.90%

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- Definition Discount rate adopted in measuring projected termination and long service obligations and net periodic benefit cost. + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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

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- Definition Rate of compensation increase adopted in measuring projected termination and long service obligations and net periodic benefit cost. + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant