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Subsequent Event - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 6 Months Ended | 1 Months Ended
--- | --- | ---
Jun. 30, 2014 | Jul. 31, 2014 Subsequent Event [Member] Servier [Member]
Subsequent Event [Line Items] |  |
Review period | 30 days | 30 days
Milestone payment due |  | $ 5.0

X
- Definition Additional Milestone Payment + References No definition available. + Details
Name: | mgnx_AdditionalMilestonePayment
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Review period by regulatory authority. + References No definition available. + Details
Name: | mgnx_ReviewPeriodByRegulatoryAuthority
Namespace Prefix: | mgnx_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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