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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
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule Of Share Repurchase Programs [Line Items] |  |  |
Shares purchased | 17,534,902 mga_CommonStockRepurchasedDuringPeriodShares | 14,082,321 mga_CommonStockRepurchasedDuringPeriodShares | 895,032 mga_CommonStockRepurchasedDuringPeriodShares
Cash amount | $ 1,766 mga_CashAmountForCommonStockRepurchasedUnderNcib | $ 1,013 mga_CashAmountForCommonStockRepurchasedUnderNcib | $ 40 mga_CashAmountForCommonStockRepurchasedUnderNcib
2011 Bid [Member] |  |  |
Schedule Of Share Repurchase Programs [Line Items] |  |  |
Maximum number of shares | 12,000,000 mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember |  |
Shares purchased |  |  | 467,630 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember
Cash amount |  |  | 21 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandElevenBidMemberMember
2012 Bid [Member] |  |  |
Schedule Of Share Repurchase Programs [Line Items] |  |  |
Maximum number of shares | 12,000,000 mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember |  |
Shares purchased |  | 11,572,598 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember | 427,402 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember
Cash amount |  | 814 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember | 19 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandTwelveBidMemberMember
2013 Bid [Member] |  |  |
Schedule Of Share Repurchase Programs [Line Items] |  |  |
Maximum number of shares | 20,000,000 mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember |  |
Shares purchased | 15,135,714 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember | 2,509,723 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember |
Cash amount | 1,525 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember | 199 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandThirteenBidMemberMember |
2014 Bid [Member] |  |  |
Schedule Of Share Repurchase Programs [Line Items] |  |  |
Maximum number of shares | 20,000,000 mga_MaximumNumberOfCommonStockSharesToBeRepurchased / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember |  |
Shares purchased | 2,399,188 mga_CommonStockRepurchasedDuringPeriodShares / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember |  |
Cash amount | $ 241 mga_CashAmountForCommonStockRepurchasedUnderNcib / us-gaap_PlanNameAxis = mga_TwoThousandFourteenBidMemberMember |  |

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