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v3.20.1

Segmented Information - Schedule of Segment Information for Reporting Segments (Parenthetical) (Detail) $ in Millions | Dec. 31, 2019 USD ($)
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Power & Vision [Member] |
Segment Reporting Information [Line Items] |
Net assets held for sale | $ 541

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- Definition Fair value portion of assets classified as held for sale. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mga_PowerAndVisionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |