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Accumulated Other Comprehensive Loss - Additional Information (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Scenario, Forecast [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amount of comprehensive loss that is expected to be reclassified to net income	$ 21

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: