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Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016
Significant Accounting Policies [Line Items]
Changes in assumptions and experience gains and losses	Greater than 10%
Period of benefit obligation in current liabilities	Next twelve months
Minimum [Member]
Significant Accounting Policies [Line Items]
Amortizable intangible assets, estimated useful life	4 years
Minimum [Member] | Building [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	2.50%
Minimum [Member] | General Purpose Equipment [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	7.00%
Minimum [Member] | Special Purpose Equipment [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	10.00%
Maximum [Member]
Significant Accounting Policies [Line Items]
Amortizable intangible assets, estimated useful life	15 years
Maximum [Member] | Building [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	5.00%
Maximum [Member] | General Purpose Equipment [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	10.00%
Maximum [Member] | Special Purpose Equipment [Member]
Significant Accounting Policies [Line Items]
Annual rates of depreciation on fixed assets	33.00%

X

- Definition

Changes in assumptions and experience gains and losses recognized in income.

+ References

No definition available.

+ Details

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

X

- Definition

Period of benefit obligation in current liabilities.

+ References

No definition available.

+ Details

Name:	mga_PeriodOfBenefitObligationInCurrentLiabilities
Namespace Prefix:	mga_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Property plant and equipment depreciation rate.

+ References

No definition available.

+ Details

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

X

- Definition

Significant Accounting Policies [Line Items]

+ References

No definition available.

+ Details

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

X

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=mga_GeneralPurposeEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=mga_SpecialPurposeEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: