Business acquisition percentage of interest acquired.

No definition available.

Name:	mga_BusinessAcquisitionPercentageOfInterestAcquired
Namespace Prefix:	mga_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Business Combination Step Acquisition Equity Interest In Acquiree Remeasurement Gain After Tax

+ References

No definition available.

+ Details

Name:	mga_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGainAfterTax
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Estimated Cost Fair Value Input

+ References

No definition available.

+ Details

Name:	mga_EstimatedCostFairValueInput
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Long lived asset impairment charges after tax.

+ References

No definition available.

+ Details

Name:	mga_LongLivedAssetsImpairmentChargesAfterTax
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage of noncontrolling interest in equity accounted investment.

+ References

No definition available.

+ Details

Name:	mga_PercentageOfNoncontrollingInterestInEquityAccountedInvestment
Namespace Prefix:	mga_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Restructuring Charges Net Of Tax

+ References

No definition available.

+ Details

Name:	mga_RestructuringChargesNetOfTax
Namespace Prefix:	mga_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

In a business combination achieved in stages, this element represents the amount of gain recognized by the entity as a result of remeasuring to fair value the equity interest in the acquiree it held before the business combination.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (g)(2)

-URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463

+ Details

Name:	us-gaap_BusinessCombinationStepAcquisitionEquityInterestInAcquireeRemeasurementGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13777-109266

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

Name:	us-gaap_GoodwillImpairmentLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount after tax of loss recognized that results from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

No definition available.

+ Details

Name:	us-gaap_GoodwillImpairmentLossNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=8077374&loc=d3e2420-110228

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391110&loc=d3e2921-110230

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 958

-SubTopic 225

-Section 45

-Paragraph 11

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28367520&loc=d3e92212-112881

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Impairment

-URI http://asc.fasb.org/extlink&oid=6515133

+ Details

Name:	us-gaap_ImpairmentOfLongLivedAssetsHeldForUse
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of a controlling interest in another entity or an entity that is related to it but not strictly controlled (for example, an unconsolidated subsidiary, affiliate, joint venture or equity method investment).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireBusinessesAndInterestInAffiliates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

+ References

+ Details

Name:	us-gaap_PaymentsToAcquireBusinessesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 420

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 420

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SAB TOPIC 5.P.4(b))

-URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140904-122747

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 420

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.P.3)

-URI http://asc.fasb.org/extlink&oid=27011515&loc=d3e140864-122747

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 5

-Section P

-Subsection 3, 4

+ Details

Name:	us-gaap_RestructuringCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=mga_ECarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=mga_SttTechnologiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_CorporateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=mga_RestOfWorldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_EuropeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=us-gaap_NorthAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Other Expense (Income), Net - Other Expense (Income), Net (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended		1 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Aug. 31, 2012	Dec. 31, 2014	Oct. 26, 2012	Dec. 31, 2012
Component of Operating Other Cost and Expense [Abstract]
Impairment of goodwill	$ 22us-gaap_GoodwillImpairmentLoss
STT Technologies Inc. [Member]
Component of Operating Other Cost and Expense [Abstract]
Company acquired					50.00%
E-Car [Member]
Component of Operating Other Cost and Expense [Abstract]
Company acquired			27.00%
Corporate [Member]
Component of Operating Other Cost and Expense [Abstract]
Estimated fair value		205
Non-cash gain		153
Non-cash gain after tax		125
Corporate [Member] \| E-Car [Member]
Component of Operating Other Cost and Expense [Abstract]
Company acquired			27.00%
Cash acquired consideration			75
Percentage of non-controlling interest in an equity accounted investment			73.00%
North America [Member]
Component of Operating Other Cost and Expense [Abstract]
Long-lived asset impairment charges	23	2		18
Long-lived asset impairment charges, after tax	11	1		12
Non-cash gain		35
North America [Member] \| STT Technologies Inc. [Member]
Component of Operating Other Cost and Expense [Abstract]
Company acquired					50.00%
Cash consideration					55
Estimated fair value		55
Non-cash gain		35
Non-cash gain after tax		35
Europe [Member]
Component of Operating Other Cost and Expense [Abstract]
Long-lived asset impairment charges		23				23
Long-lived asset impairment charges, after tax						22
Restructuring charges	89	55		46
Restructuring charges, after tax	64	53		41
Rest of World [Member]
Component of Operating Other Cost and Expense [Abstract]
Long-lived asset impairment charges	10
Long-lived asset impairment charges, after tax	10
Impairment of goodwill	22
Impairment of goodwill, after tax	$ 22