Lessee, Operating Lease, Lease Not yet Commenced, Liability.

No definition available.

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Namespace Prefix:	mga_
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Balance Type:	credit
Period Type:	instant

- Definition

Operating Lease, Year Of Commencement

+ References

No definition available.

+ Details

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Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Description of terms and conditions of option to extend lessee's operating lease. Includes, but is not limited to, information about option recognized as part of right-of-use asset and lease liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

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

- Definition

Description of terms and conditions of option to terminate lessee's operating lease. Includes, but is not limited to, information about option recognized as part of right-of-use asset and lease liability.

+ References

+ Details

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

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of operating lease expense. Excludes sublease income.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918638-209977

+ Details

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

- Details

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- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

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- Details

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Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Operating Lease Option To Extend	options to extend the term of the lease for up to 12 years
Operating Lease Option To Terminate	terminate the lease within 1 year
Cost of Sales [Member]
Lessee, Lease, Description [Line Items]
Operating lease expense		$ 330
Selling, General and Administrative Expenses [Member]
Lessee, Lease, Description [Line Items]
Operating lease expense		$ 31
Manufacturing Facility [Member]
Lessee, Lease, Description [Line Items]
Operating lease liabilitites	$ 22
Operating lease year of commencement	2020
Use Rights [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	90 years
Maximum [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	33 years
Maximum [Member] \| Manufacturing Facility [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	8 years
Minimum [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	1 year
Minimum [Member] \| Manufacturing Facility [Member]
Lessee, Lease, Description [Line Items]
Operating lease term	1 year