Accumulated Other Comprehensive Loss - Additional Information (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Equity [Abstract]
Amount of comprehensive loss that is expected to be reclassified to net income	$ 44

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

No definition available.

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