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Stock-Based Compensation - Schedule of Options Outstanding (Parenthetical) (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
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Total intrinsic value of options exercised | $ 85 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 56 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 6 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Stock Option Holder [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Exercised number of shares, under stock option plan |  | 849,999 mga_CashlessExerciseOfStockOptionsShares / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember | 1,100,001 mga_CashlessExerciseOfStockOptionsShares / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember
Cash payments made on cashless exercise of stock options |  | $ 23 mga_CashPaymentsMadeOnCashlessExerciseOfStockOptions / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember | $ 19 mga_CashPaymentsMadeOnCashlessExerciseOfStockOptions / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember

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- Definition Cashless Exercise Of Stock Options Shares + References No definition available. + Details
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- Definition Cash payments made on cashless exercise of stock options. + References No definition available. + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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