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Subsequent Events - Additional Information (Detail) (Subsequent Event [Member], USD $) In Millions, unless otherwise specified | 0 Months Ended
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Feb. 28, 2014
Subsequent Event [Line Items] |
Income tax payable period | 3 years
Minimum [Member] |
Subsequent Event [Line Items] |
Tax expense | $ 25
Maximum [Member] |
Subsequent Event [Line Items] |
Tax expense | $ 30

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- Definition Subsequent Event Income Tax Payable Period + References No definition available. + Details
Name: | mga_SubsequentEventIncomeTaxPayablePeriod
Namespace Prefix: | mga_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Monetary effect of a change in the tax status effective after year-end but before the financial statements for the reporting period are issued or available to be issue by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32590-109319 + Details
Name: | us-gaap_SubsequentEventEffectOfChangeInTaxStatus
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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