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Employee Benefit Plan - Additional Information (Detail) (USD $) | 0 Months Ended | 12 Months Ended
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Sep. 25, 2002 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract] |  |  |  |
Eligibility of employee participation Plan | 21 years |  |  |
Percentage of employee contribution to salary | 100.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent |  |  |
Percentage of employee vested contribution |  | 100.00%mgnx_DefinedContributionPlanEmployeesContributionAnnualVestingPercentage |  |
Defined contribution plan, employer discretionary contribution amount |  | $ 318,390 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | $ 252,930 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | $ 225,195 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount

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- Definition Defined contribution plan, employees contribution, annual vesting percentage. + References No definition available. + Details
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Namespace Prefix: | mgnx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Employees eligible age to participate in benefit plan. + References No definition available. + Details
Name: | mgnx_EmployeesEligibleAgeToParticipateInBenefitPlan
Namespace Prefix: | mgnx_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_DefinedContributionPensionAndOtherPostretirementPlansDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration