Financial Instruments - Schedule of Company's Financial Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Trading
Trading Securities	$ 1,341us-gaap_TradingSecurities	$ 1,646us-gaap_TradingSecurities
Held-to-maturity investments
Severance investments	4us-gaap_HeldToMaturitySecurities	5us-gaap_HeldToMaturitySecurities
Available-for-sale investments
Equity investments	5us-gaap_AvailableForSaleSecurities	4us-gaap_AvailableForSaleSecurities
Loans and receivables
Accounts receivable	5,635us-gaap_AccountsReceivableNet	5,246us-gaap_AccountsReceivableNet
Long-term receivables included in other assets [note 12]	87	111
Total	5,722mga_AccountsAndLongTermReceivables	5,357mga_AccountsAndLongTermReceivables
Other financial liabilities
Bank indebtedness	33us-gaap_ShortTermBorrowings	41us-gaap_ShortTermBorrowings
Long-term debt (including portion due within one year)	995us-gaap_LongTermDebt	332us-gaap_LongTermDebt
Accounts payable	5,105	4,781
Total	6,133us-gaap_OtherLiabilities	5,154us-gaap_OtherLiabilities
Derivatives designated as effective hedges, measured at fair value, Assets	30	62
Derivatives designated as effective hedges, measured at fair value, Total	(146)	(4)
Derivatives designated as effective hedges, measured at fair value, Liabilities	(174)	(65)
Foreign Exchange Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Total	(145)	(3)
Prepaid Expenses and Other [Member] \| Foreign Exchange Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Assets	22	42
Other Assets [Member] \| Foreign Exchange Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Assets	8	20
Other Accrued Liabilities [Member] \| Foreign Exchange Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Liabilities	(93)	(37)
Other Accrued Liabilities [Member] \| Commodity Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Liabilities	(1)	(1)
Other Noncurrent Liabilities [Member] \| Foreign Exchange Contract [Member]
Other financial liabilities
Derivatives designated as effective hedges, measured at fair value, Liabilities	(82)	(28)
Cash and Cash Equivalents [Member]
Trading
Trading Securities	1,253	1,554
Asset-backed Securities [Member]
Trading
Trading Securities	$ 88	$ 92

Accounts and long term receivables.

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