Other Assets - Summary of Other Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule Of Other Assets [Line Items]
Preproduction costs related to long-term supply agreements with contractual guarantee for reimbursement	$ 297	$ 301
Long-term receivables [note 20]	95	176
Patents and licenses, net	34	30
Unrealized gain on cash flow hedges [note 20]	32	15
E-Car intangible [note 6]	158	0
Other, net [note 6]	137	72
Total other assets	$ 753	$ 594

Other Intangible Assets

No definition available.

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

- Details

Name:	mga_ScheduleOfOtherAssetsLineItems
Namespace Prefix:	mga_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Unrealized gain on cash flow hedges.

+ References

No definition available.

+ Details

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

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=7658586&loc=d3e16323-109275

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 45

-Subparagraph a(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

An amount representing an agreement for an unconditional promise by the maker to pay the Entity (holder) a definite sum of money at a future date more than one year from the balance sheet date, net of any write-downs taken for collection uncertainty on the part of the holder. Such amount may include accrued interest receivable in accordance with the terms of the debt. The debt also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. This amount does not include amounts related to receivables held-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

+ Details

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

- Definition

The aggregate carrying amounts, as of the balance sheet date, of assets not separately disclosed in the balance sheet.

+ References

+ Details

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

- Definition

The sum of the carrying amounts, as of the balance sheet date, of assets which have not been itemized or categorized in the footnotes to the financial statements and are a component of Other Assets (OtherAssets).

+ References

No definition available.

+ Details

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

- Definition

The carrying amount as of the balance sheet date of capitalized preproduction costs incurred on long-term supply arrangements, including recoverable costs to design and develop products.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 340

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6387057&loc=d3e6169-108318

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 340

-SubTopic 10

-Section 25

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6387057&loc=d3e6233-108318

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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 99-5

-Paragraph 6

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 340

-SubTopic 10

-Section S99

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6955238&loc=d3e105117-122735

+ Details

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