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Stock-Based Compensation - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2014 Incentive_Stock_Option_Plan | Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Number of incentive stock option plans | 2 mga_NumberOfIncentiveStockOptionPlans |
Holding period of restricted stock available to executives | 4 years |
Share based compensation arrangement long-term retention program restricted stock available to executive releasing period | 10 years |
Number of days considered for weighted average trading price | 20 days |
Percentage of annual retainers | 60.00%mga_PercentageOfAnnualRetainersForDirectorShareBasedCompensationPlan |
Percentage of additional annual retainers | 100.00%mga_PercentageOfAnnualRetainersDefermentForDirectorShareBasedCompensationPlanMaximum |
Stock Option Plan 1987 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Period of granted option |  | 7 years
Stock Option Plan 2009 [Member] |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |
Maximum number of shares, issued under the option plan | 16,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_AwardTypeAxis = mga_StockOptionPlanTwoThousandNineMember |
Number of shares available to be granted | 6,793,030 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_AwardTypeAxis = mga_StockOptionPlanTwoThousandNineMember | 7,516,831 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_AwardTypeAxis = mga_StockOptionPlanTwoThousandNineMember
Period of granted option | 7 years |
Option vested under stock option plan to employees and consultants | One-third on each of the first three anniversaries of the date of grant. |

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- Definition Percentage of annual retainers deferment for director share based compensation plan maximum. + References No definition available. + Details
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- Definition Percentage of annual retainers for director share based compensation plan. + References No definition available. + Details
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- Definition Share based compensation arrangement long term retention program restricted stock available to executive holding period. + References No definition available. + Details
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- Definition Share based compensation arrangement long term retention program restricted stock available to executive releasing period. + References No definition available. + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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