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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
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 730,476mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased 882,988mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased 1,026,304mga_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 587,324mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased 730,476mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased 882,988mga_ShareBasedCompensationArrangementLongTermRetentionProgramCommonSharesAwardedNotReleased
Stated value, Awarded and not released, Beginning balance $ 25mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares $ 30mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares $ 35mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares
Stated value, Release of restricted stock (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram (5)mga_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeituresLongtermRetentionProgram
Stated value, Awarded and not released, Ending balance $ 20mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares $ 25mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares $ 30mga_ShareBasedCompensationArrangementLongTermRetentionProgramReductionInStatedValueOfCommonShares