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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 | $ 2,250,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity |  |  |  |
Revolving credit facility maturity date | Jun. 20, 2018 |  |  |  |
Revolving credit facility extended maturity date |  | 2019-06-20 |  |  |
Interest paid in cash |  | 45,000,000 us-gaap_InterestPaid | 32,000,000 us-gaap_InterestPaid | 32,000,000 us-gaap_InterestPaid |
Operating lease expense |  | 344,000,000 us-gaap_OperatingLeasesRentExpenseNet | 363,000,000 us-gaap_OperatingLeasesRentExpenseNet | 325,000,000 us-gaap_OperatingLeasesRentExpenseNet |
Founder and Affiliated Entity [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Business development, consulting, other business services agreement expenses |  | 57,000,000 mga_BusinessDevelopmentConsultingAndOtherExpenses / us-gaap_StatementScenarioAxis = mga_FounderAndAffiliatedEntityMember | 52,000,000 mga_BusinessDevelopmentConsultingAndOtherExpenses / us-gaap_StatementScenarioAxis = mga_FounderAndAffiliatedEntityMember | 47,000,000 mga_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 |  |  |  |  | 750,000,000 us-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 | 200,000,000 mga_LineOfCreditFacilityFullyTransferableAmount / us-gaap_LongtermDebtTypeAxis = mga_AsianTrancheMember |  |  |  |
Mexican Tranche [Member] |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Line of credit facility fully transferable amount | $ 50,000,000 mga_LineOfCreditFacilityFullyTransferableAmount / us-gaap_LongtermDebtTypeAxis = mga_MexicanTrancheMember |  |  |  |

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- Definition Business Development Consulting And Other Expenses + References No definition available. + Details
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- Definition Line of credit facility fully transferable amount. + References No definition available. + Details
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- Definition Revolving Credit Facility Expiration Date + References No definition available. + Details
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- Definition Revolving Credit Facility Extended Maturity Date + References No definition available. + Details
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- 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 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
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- 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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- Definition Date the debt instrument was issued, in CCYY-MM-DD format. + 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- 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 -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)(2)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717 + Details
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