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Debt and Commitments - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	May 16, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 16, 2014
Debt Instrument [Line Items]
Borrowings under revolving credit facility	$ 2,250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Revolving credit facility maturity date	Jun. 20, 2018
Revolving credit facility extended maturity date		2019-06-20
Interest paid in cash		45,000,000us-gaap_InterestPaid	32,000,000us-gaap_InterestPaid	32,000,000us-gaap_InterestPaid
Operating lease expense		344,000,000us-gaap_OperatingLeasesRentExpenseNet	363,000,000us-gaap_OperatingLeasesRentExpenseNet	325,000,000us-gaap_OperatingLeasesRentExpenseNet
Founder and Affiliated Entity [Member]
Debt Instrument [Line Items]
Business development, consulting, other business services agreement expenses		57,000,000mga_BusinessDevelopmentConsultingAndOtherExpenses / us-gaap_StatementScenarioAxis = mga_FounderAndAffiliatedEntityMember	52,000,000mga_BusinessDevelopmentConsultingAndOtherExpenses / us-gaap_StatementScenarioAxis = mga_FounderAndAffiliatedEntityMember	47,000,000mga_BusinessDevelopmentConsultingAndOtherExpenses / us-gaap_StatementScenarioAxis = mga_FounderAndAffiliatedEntityMember
3.625% Fixed-Rate Senior Notes [Member]
Debt Instrument [Line Items]
Debt issuance date		Jun. 16, 2014
Debt issued amount					750,000,000us-gaap_DebtInstrumentFaceAmount / us-gaap_LongtermDebtTypeAxis = mga_ThreePointSixTwoFiveSeniorNotesMember
Interest rate					3.625%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = mga_ThreePointSixTwoFiveSeniorNotesMember
Debt maturity date		Jun. 15, 2024
Interest payment terms		Interest is payable on June 15 and December 15 of each year
Asian Tranche [Member]
Debt Instrument [Line Items]
Line of credit facility fully transferable amount	200,000,000mga_LineOfCreditFacilityFullyTransferableAmount / us-gaap_LongtermDebtTypeAxis = mga_AsianTrancheMember
Mexican Tranche [Member]
Debt Instrument [Line Items]
Line of credit facility fully transferable amount	$ 50,000,000mga_LineOfCreditFacilityFullyTransferableAmount / us-gaap_LongtermDebtTypeAxis = mga_MexicanTrancheMember

X

- Definition

Business Development Consulting And Other Expenses

+ References

No definition available.

+ Details

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Balance Type:	debit
Period Type:	duration

X

- Definition

Line of credit facility fully transferable amount.

+ References

No definition available.

+ Details

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X

- Definition

Revolving Credit Facility Expiration Date

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

Revolving Credit Facility Extended Maturity Date

+ References

No definition available.

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X

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

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-Name Accounting Standards Codification

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X

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

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X

- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ References

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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- Details

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X

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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- Definition

Description of the payment terms of the debt instrument (for example, whether periodic payments include principal and frequency of payments) and discussion about any contingencies associated with the payment.

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

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X

- Definition

The amount of cash paid for interest during the period.

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-Name Accounting Standards Codification

-Topic 230

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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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-Publisher FASB

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X

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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