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Debt and Commitments - Schedule of Company's Long-Term Debt (Parenthetical) (Detail)	Dec. 31, 2014	Dec. 31, 2013
Senior Notes [Member]
Debt Instrument [Line Items]
Interest rate stated percentage	3.625%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_LongtermDebtTypeAxis = us-gaap_SeniorNotesMember
Bank Term Debt [Member]
Debt Instrument [Line Items]
Weighted average interest rate of bank term debt	8.20%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = mga_BankTermDebtMember	6.30%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = mga_BankTermDebtMember
Government Loans [Member]
Debt Instrument [Line Items]
Weighted average interest rate of bank term debt	5.50%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = mga_GovernmentLoansMember	5.90%us-gaap_LongtermDebtWeightedAverageInterestRate / us-gaap_LongtermDebtTypeAxis = mga_GovernmentLoansMember

X

- Definition

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

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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X

- Details

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

X

- Definition

Weighted average interest rate of long-term debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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X

- Details

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Balance Type:
Period Type:

X

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
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Balance Type:
Period Type: