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Capital Stock - Summary of Normal Course Issuer Bids (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Share Repurchase Programs [Line Items]
Shares purchased	17,534,902mga_CommonStockRepurchasedDuringPeriodShares	14,082,321mga_CommonStockRepurchasedDuringPeriodShares	895,032mga_CommonStockRepurchasedDuringPeriodShares
Cash amount	$ 1,766mga_CashAmountForCommonStockRepurchasedUnderNcib	$ 1,013mga_CashAmountForCommonStockRepurchasedUnderNcib	$ 40mga_CashAmountForCommonStockRepurchasedUnderNcib
2011 Bid [Member]
Schedule Of Share Repurchase Programs [Line Items]
Maximum number of shares	12,000,000mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember
Shares purchased			467,630mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember
Cash amount			21mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember
2012 Bid [Member]
Schedule Of Share Repurchase Programs [Line Items]
Maximum number of shares	12,000,000mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember
Shares purchased		11,572,598mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember	427,402mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember
Cash amount		814mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember	19mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember
2013 Bid [Member]
Schedule Of Share Repurchase Programs [Line Items]
Maximum number of shares	20,000,000mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember
Shares purchased	15,135,714mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember	2,509,723mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember
Cash amount	1,525mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember	199mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember
2014 Bid [Member]
Schedule Of Share Repurchase Programs [Line Items]
Maximum number of shares	20,000,000mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember
Shares purchased	2,399,188mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember
Cash amount	$ 241mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember

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