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v2.4.1.9

Capital Stock - Maximum Number of Shares Outstanding Exercised or Converted (Detail) (Convertible Common Stock [Member], March 5, 2015 [Member]) | Dec. 31, 2014
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Convertible Common Stock [Member] | March 5, 2015 [Member] |
Class of Stock [Line Items] |
Common Shares | 205,179,261 us-gaap_CommonStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_ConvertibleCommonStockMember / us-gaap_StatementScenarioAxis = mga_MarchFifthTwoThousandFifteenMember
Stock options | 4,898,935 mga_PotentiallyDilutiveSecuritiesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_ConvertibleCommonStockMember / us-gaap_StatementScenarioAxis = mga_MarchFifthTwoThousandFifteenMember
Total number of shares outstanding | 210,078,196 mga_CommonSharesAndPotentiallyDilutiveSecuritiesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_ConvertibleCommonStockMember / us-gaap_StatementScenarioAxis = mga_MarchFifthTwoThousandFifteenMember

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- Definition Common shares and potentially dilutive securities outstanding. + References No definition available. + Details
Name: | mga_CommonSharesAndPotentiallyDilutiveSecuritiesOutstanding
Namespace Prefix: | mga_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Potentially dilutive securities outstanding. + References No definition available. + Details
Name: | mga_PotentiallyDilutiveSecuritiesOutstanding
Namespace Prefix: | mga_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.29) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 + Details
Name: | us-gaap_CommonStockSharesOutstanding
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant