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Accumulated Other Comprehensive Loss - Additional Information (Detail) $ in Millions	Dec. 31, 2020 USD ($)
Equity [Abstract]
Amount of comprehensive loss that is expected to be reclassified to net income	$ 44

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

- References

No definition available.

+ Details

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