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Leases - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($)
Lessee, Lease, Description [Line Items] |
Operating lease term | 5 years
Operating Lease Option To Extend | options to extend the term of the lease
Manufacturing Facility [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease liabilitites | $ 12
Operating lease year of commencement | 2021
Use Rights [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease term | 90 years
Maximum [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease term | 7 years
Maximum [Member] | Manufacturing Facility [Member] |
Lessee, Lease, Description [Line Items] |
Operating lease term | 10 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

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- Definition Lessee, Operating Lease, Lease Not yet Commenced, Liability. + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Operating Lease, Year Of Commencement + References No definition available. + Details
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Namespace Prefix: | mga_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

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- 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_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- 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=121609121&loc=SL77918673-209980 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- 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=121609121&loc=SL77918673-209980 + Details
Name: | us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

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

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

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |