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Employee Equity and Profit Participation Program - Additional Information (Detail) - Trust for Benefit of Employees [Member] - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount borrowed from company to purchase common shares maximum	$ 10,000,000	$ 55,000,000
Amount included in accounts receivable	$ 10,000,000	$ 5,000,000

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- Definition

Amount borrowed from company to purchase common shares maximum.

+ References

No definition available.

+ Details

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

X

- Definition

Trust Indebtedness Receivable

+ References

No definition available.

+ Details

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

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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
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Period Type:	duration

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: