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Stock-Based Compensation - Schedule of Reduction in Stated Value of Company's Common Shares (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |  |  |
Number of shares, Awarded and not released, Beginning balance | 730,476 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased | 882,988 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased | 1,026,304 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased
Number of shares, Release of restricted stock | (143,152)us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures | (152,512)us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures | (143,316)us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
Number of shares, Awarded and not released, Ending balance | 587,324 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased | 730,476 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased | 882,988 mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased
Stated value, Awarded and not released, Beginning balance | $ 25 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares | $ 30 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares | $ 35 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares
Stated value, Release of restricted stock | (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram | (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram | (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram
Stated value, Awarded and not released, Ending balance | $ 20 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares | $ 25 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares | $ 30 mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares

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- Definition Share based compensation arrangement long term retention program common shares awarded not released. + References No definition available. + Details
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- Definition Share based compensation arrangement long term retention program reduction in stated value of common shares. + References No definition available. + Details
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- Definition Value of stock related to Restricted Stock Awards issued during the period, net of stock value of such awards forfeited, related to long-term retention program. + References No definition available. + Details
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- Details
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- Definition Number of shares issued during the period related to Restricted Stock Awards, net of any shares forfeited. + 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30 -Article 5 + Details
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