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Stock-Based Compensation - Summary of Stock-Based Compensation Expense Recorded in Selling, General and Administrative Expenses (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Incentive Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 40us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember	$ 40us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember	$ 42us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember
Selling, General and Administrative Expenses [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	40us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember	35us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember	38us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember
Selling, General and Administrative Expenses [Member] | Incentive Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	15us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember	15us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember	19us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_IncentiveStockOptionPlanMember
Selling, General and Administrative Expenses [Member] | Long-term Retention [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	4us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_LongTermRetentionPlanMember	4us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_LongTermRetentionPlanMember	5us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_LongTermRetentionPlanMember
Selling, General and Administrative Expenses [Member] | Restricted Stock Unit [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	21us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_RestrictedStockUnitMember	16us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_RestrictedStockUnitMember	14us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_RestrictedStockUnitMember
Selling, General and Administrative Expenses [Member] | Fair Value Adjustment for Liability Classified DSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense		$ 5us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_FairValueAdjustmentMember	$ 4us-gaap_AllocatedShareBasedCompensationExpense / us-gaap_IncomeStatementLocationAxis = us-gaap_SellingGeneralAndAdministrativeExpensesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mga_FairValueAdjustmentMember

X

- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.F)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (h)(1)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

+ Details

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Balance Type:	debit
Period Type:	duration

X

- Details

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Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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