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Segmented Information - Additional Information (Detail)	Dec. 31, 2017 Market
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