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Financial Instruments - Foreign Exchange Forward Contracts Representing Commitments to Buy and Sell Various Foreign Currencies (Detail) In Millions, unless otherwise specified |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  | 12 Months Ended |
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Dec. 31, 2013 Canadian Dollar [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] U S Dollar [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] Euro [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] Long [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] Long [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] Short [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2014 [Member] Short [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] U S Dollar [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] Euro [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] Long [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] Long [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] Short [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2015 [Member] Short [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] U S Dollar [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] Euro [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] Long [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] Long [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] Short [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2016 [Member] Short [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2017 [Member] U S Dollar [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2017 [Member] Euro [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2017 [Member] Long [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2017 [Member] Long [Member] EUR (€) | Dec. 31, 2013 Canadian Dollar [Member] 2018 [Member] U S Dollar [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2018 [Member] Euro [Member] | Dec. 31, 2013 Canadian Dollar [Member] 2018 [Member] Short [Member] USD ($) | Dec. 31, 2013 Canadian Dollar [Member] 2018 [Member] Short [Member] EUR (€) | Dec. 31, 2013 U S Dollar [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2014 [Member] Mexican Peso [Member] | Dec. 31, 2013 U S Dollar [Member] 2014 [Member] Long [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2014 [Member] Short [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2015 [Member] Mexican Peso [Member] | Dec. 31, 2013 U S Dollar [Member] 2015 [Member] Long [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2015 [Member] Short [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2016 [Member] Mexican Peso [Member] | Dec. 31, 2013 U S Dollar [Member] 2016 [Member] Long [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2016 [Member] Short [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2017 [Member] Mexican Peso [Member] | Dec. 31, 2013 U S Dollar [Member] 2017 [Member] Long [Member] MXN | Dec. 31, 2013 U S Dollar [Member] 2018 [Member] Mexican Peso [Member] | Dec. 31, 2013 U S Dollar [Member] 2018 [Member] Short [Member] MXN | Dec. 31, 2013 Euros [Member] USD ($) | Dec. 31, 2013 Euros [Member] CZK | Dec. 31, 2013 Euros [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2014 [Member] U S Dollar [Member] | Dec. 31, 2013 Euros [Member] 2014 [Member] British Pound [Member] | Dec. 31, 2013 Euros [Member] 2014 [Member] Czech Koruna [Member] | Dec. 31, 2013 Euros [Member] 2014 [Member] Long [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2014 [Member] Long [Member] CZK | Dec. 31, 2013 Euros [Member] 2014 [Member] Long [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2014 [Member] Short [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2014 [Member] Short [Member] CZK | Dec. 31, 2013 Euros [Member] 2014 [Member] Short [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2015 [Member] U S Dollar [Member] | Dec. 31, 2013 Euros [Member] 2015 [Member] British Pound [Member] | Dec. 31, 2013 Euros [Member] 2015 [Member] Czech Koruna [Member] | Dec. 31, 2013 Euros [Member] 2015 [Member] Long [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2015 [Member] Long [Member] CZK | Dec. 31, 2013 Euros [Member] 2015 [Member] Long [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2015 [Member] Short [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2015 [Member] Short [Member] CZK | Dec. 31, 2013 Euros [Member] 2015 [Member] Short [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2016 [Member] U S Dollar [Member] | Dec. 31, 2013 Euros [Member] 2016 [Member] British Pound [Member] | Dec. 31, 2013 Euros [Member] 2016 [Member] Czech Koruna [Member] | Dec. 31, 2013 Euros [Member] 2016 [Member] Long [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2016 [Member] Long [Member] CZK | Dec. 31, 2013 Euros [Member] 2016 [Member] Long [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2016 [Member] Short [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2016 [Member] Short [Member] CZK | Dec. 31, 2013 Euros [Member] 2016 [Member] Short [Member] GBP (£) | Dec. 31, 2013 Euros [Member] 2017 [Member] U S Dollar [Member] | Dec. 31, 2013 Euros [Member] 2017 [Member] British Pound [Member] | Dec. 31, 2013 Euros [Member] 2017 [Member] Czech Koruna [Member] | Dec. 31, 2013 Euros [Member] 2017 [Member] Short [Member] USD ($) | Dec. 31, 2013 Euros [Member] 2017 [Member] Short [Member] CZK | Dec. 31, 2013 Euros [Member] 2017 [Member] Short [Member] GBP (£)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Foreign exchange forward contract, Purchases Exchange Rate |  |  | 1.05974 | 1.34745 |  |  |  |  | 1.04289 | 1.44078 |  |  |  |  | 1.03766 | 0.00000 |  |  |  |  | 1.07747 | 0.00000 |  |  |  |  |  |  |  | 0.07128 |  |  | 0.07170 |  |  | 0.07166 |  |  | 0.00000 |  |  |  |  |  |  | 0.74048 | 1.14482 | 0.03956 |  |  |  |  |  |  | 0.74375 | 1.19170 | 0.03945 |  |  |  |  |  |  | 0.73702 | 0.00000 | 0.03904 |  |  |  |  |  |  |  |  |  |  |  |
Foreign exchange forward contract, Sales Exchange Rate |  |  | 1.03832 | 1.40577 |  |  |  |  | 1.04592 | 1.42922 |  |  |  |  | 1.06735 | 0.00000 |  |  |  |  |  |  |  |  | 1.08040 | 0.00000 |  |  |  | 0.07678 |  |  | 0.00000 |  |  | 0.00000 |  |  |  |  | 0.00000 |  |  |  |  | 0.75973 | 1.16896 | 0.03890 |  |  |  |  |  |  | 0.75549 | 1.16338 | 0.00000 |  |  |  |  |  |  | 0.75060 | 1.17217 | 0.00000 |  |  |  |  |  |  | 0.73141 | 0.00000 | 0.00000 |  |  |
Notional amount of foreign currency derivatives | $ (983) | € 46 |  |  | $ 206 | € 47 | $ 656 | € 12 |  |  | $ 54 | € 12 | $ 332 | € 1 |  |  | $ 17 | € 0 | $ 185 | € 0 |  |  | $ 85 | € 0 |  |  | $ 2 | € 0 | 6,191 |  | 3,509 | 2 |  | 2,174 | 0 |  | 510 | 0 |  | 0 |  | 0 | $ (208) | 3,624 | £ (14) |  |  |  | $ 37 | 2,142 | £ 31 | $ 123 | 8 | £ 42 |  |  |  | $ 6 | 1,303 | £ 13 | $ 80 | 0 | £ 15 |  |  |  | $ 2 | 187 | £ 0 | $ 34 | 0 | £ 1 |  |  |  | $ 16 | 0 | £ 0

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
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- Definition Foreign currency forward purchase contract weighted average rate. + References No definition available. + Details
Name: | mga_ForeignCurrencyForwardPurchaseContractWeightedAverageRate
Namespace Prefix: | mga_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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- Definition Foreign currency forward sale contract weighted average rate. + References No definition available. + Details
Name: | mga_ForeignCurrencyForwardSaleContractWeightedAverageRate
Namespace Prefix: | mga_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration