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Employee Benefit Plan (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2012
Retirement Benefits [Abstract] |  |  |  |
Eligible age to participate in plan |  |  |  | 21 years
Percentage of employee contribution to salary |  |  |  | 100.00%
Percentage of employee vested contribution | 100.00% |  |  |
Defined contribution plan, employer discretionary contribution amount | $ 1.1 | $ 1.0 | $ 0.4 |

X
- Definition Percentage vested of employee's contributions to defined contribution plan + References No definition available. + Details
Name: | mgnx_DefinedContributionPlanEmployeesContributionVestedPercentage
Namespace Prefix: | mgnx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Refers to minimum eligible age of employee 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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- References No definition available. + Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- 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

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