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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%

X

- 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

X

- Details

Name:	mga_PensionPlansPostretirementAndOtherEmployeeBenefitsLineItems
Namespace Prefix:	mga_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Rate of compensation increase adopted in measuring projected termination and long service obligations and net periodic benefit cost.

+ References

No definition available.

+ Details

Name:	mga_RateOfCompensationIncreaseAdoptedInMeasuringProjectedTerminationAndLongServiceObligationsAndNetPeriodicBenefitCost
Namespace Prefix:	mga_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant