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Accumulated Other Comprehensive (Loss) Income - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014
Equity [Abstract]
Amount of other comprehensive income is expected to be reclassified to net income during 2014	$ 55mga_OtherComprehensiveIncomeToBeReclassifiedToNetIncome
Amount of other comprehensive income net of income tax during 2014	$ 21mga_IncomeTaxOfOtherComprehensiveIncomeToBeReclassifiedToNetIncome

X

- Definition

Income Tax portion of other comprehensive income that is expected to be reclassified to net income over the next fiscal year.

+ References

No definition available.

+ Details

Name:	mga_IncomeTaxOfOtherComprehensiveIncomeToBeReclassifiedToNetIncome
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Other comprehensive income that is expected to be reclassified to net income over the next fiscal year, net of tax.

+ References

No definition available.

+ Details

Name:	mga_OtherComprehensiveIncomeToBeReclassifiedToNetIncome
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_EquityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration