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Income Taxes - Summary of Changes in Gross Unrecognized Tax Benefits (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Balance, beginning of year	$ 238us-gaap_UnrecognizedTaxBenefits	$ 279us-gaap_UnrecognizedTaxBenefits	$ 252us-gaap_UnrecognizedTaxBenefits
Increase based on tax positions related to current year	21us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	35us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions	68us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Decrease based on tax positions of prior years	(23)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	(44)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions	(31)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Settlements	(8)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	(24)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities	(10)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Statute expirations	(10)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	(7)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations	(5)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Foreign currency translation	(16)mga_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromForeignCurrencyTranslation	(1)mga_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromForeignCurrencyTranslation	5mga_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromForeignCurrencyTranslation
Balance, ending of year	$ 202us-gaap_UnrecognizedTaxBenefits	$ 238us-gaap_UnrecognizedTaxBenefits	$ 279us-gaap_UnrecognizedTaxBenefits

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- Definition

Unrecognized tax benefits increase decrease resulting from foreign currency translation.

+ References

No definition available.

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- Definition

Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Unrecognized Tax Benefit

-URI http://asc.fasb.org/extlink&oid=6527854



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Section 50

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- Definition

Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15A

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319

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