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Segmented Information - Additional Information (Detail) | Dec. 31, 2017 Market
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Segment Reporting [Abstract] |
Number of primary markets | 2

X
- Definition Number of primary markets in which entity operates. + References No definition available. + Details
Name: | mga_NumberOfPrimaryMarketsInWhichEntityOperates
Namespace Prefix: | mga_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_SegmentReportingAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration