Change in in noncash working capital due to acquisition.

No definition available.

Name:	mga_BusinessAcquisitionPurchasePriceAllocationChangeInNonCashWorkingCapital
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Business acquisition purchase price allocation intangibles.

+ References

No definition available.

+ Details

Name:	mga_BusinessAcquisitionPurchasePriceAllocationIntangibles
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business acquisition purchase price allocation investments.

+ References

No definition available.

+ Details

Name:	mga_BusinessAcquisitionPurchasePriceAllocationInvestments
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business acquisition purchase price allocation projected benefit obligation asset and other noncurrent liabilities.

+ References

No definition available.

+ Details

Name:	mga_BusinessAcquisitionPurchasePriceAllocationProjectedBenefitObligationAssetAndOtherNoncurrentLiabilities
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Business acquisitions purchase price allocation goodwill amount.

+ References

No definition available.

+ Details

Name:	mga_BusinessAcquisitionsPurchasePriceAllocationGoodwillAmount
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of net assets excluding cash.

+ References

No definition available.

+ Details

Name:	mga_FairValueOfNetAssetsExcludingCash
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the fair value of the noncontrolling interest in the acquiree at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of deferred tax asset attributable to deductible temporary differences and carryforwards that are expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of deferred tax liability attributable to taxable temporary differences due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

+ References

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of property, plant, and equipment recognized as of the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=39739317&loc=d3e4845-128472

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 55

-Paragraph 37

-URI http://asc.fasb.org/extlink&oid=35744172&loc=d3e2207-128464

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_ChangeInAccountingEstimateByTypeAxis=mga_PreliminaryPurchasePriceAllocationMember
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- Details

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Business Acquisitions - Net Effect of Acquisitions on Consolidation Balance and Adjustments to Preliminary Purchase Price Allocations (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Acquisition [Line Items]
Fixed assets	$ 21	$ 5
Other assets	4	2
Long-term debt	(4)
Other long-term liabilities		(2)
Deferred tax liabilities	(1)
2012 Preliminary Allocation [Member]
Business Acquisition [Line Items]
Non-cash working capital			(129)
Investments			3
Fixed assets			501
Goodwill			289
Other assets			94
Purchase intangibles			215
Long-term employee benefit liabilities			(49)
Long-term debt			(25)
Other long-term liabilities			(35)
Deferred tax liabilities			(68)
Non-controlling interests			(11)
Fair value of net assets (excluding cash)			785
2013 Adjustments [Member]
Business Acquisition [Line Items]
Non-cash working capital		(47)
Investments		(3)
Fixed assets		(36)
Goodwill		(2)
Other assets		99
Deferred tax assets		5
Long-term employee benefit liabilities		1
Long-term debt		(2)
Deferred tax liabilities		(15)
Final Allocation [Member]
Business Acquisition [Line Items]
Non-cash working capital		(176)
Fixed assets		465
Goodwill		287
Other assets		193
Deferred tax assets		5
Purchase intangibles		215
Long-term employee benefit liabilities		(48)
Long-term debt		(27)
Other long-term liabilities		(35)
Deferred tax liabilities		(83)
Non-controlling interests		(11)
Fair value of net assets (excluding cash)		$ 785