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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
	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	$ 85us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	$ 56us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	$ 6us-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		849,999mga_CashlessExerciseOfStockOptionsShares / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember	1,100,001mga_CashlessExerciseOfStockOptionsShares / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember
Cash payments made on cashless exercise of stock options		$ 23mga_CashPaymentsMadeOnCashlessExerciseOfStockOptions / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember	$ 19mga_CashPaymentsMadeOnCashlessExerciseOfStockOptions / us-gaap_AwardTypeAxis = mga_StockOptionHolderMember

X

- Definition

Cashless Exercise Of Stock Options Shares

+ References

No definition available.

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- Definition

Cash payments made on cashless exercise of stock options.

+ References

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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

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