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Subsequent Events - Narrative (Details) - Subsequent Event - I-Mab | Jul. 09, 2019 USD ($)
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Subsequent Event [Line Items] |
Non-refundable upfront payment | $ 15,000,000.0
Potential development and regulatory milestone payments under agreement | $ 135,000,000.0

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- References No definition available. + Details
Name: | mgnx_NonRefundableUpfrontFees
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
Name: | mgnx_PotentialDevelopmentandRegulatoryMilestonePaymentsUnderAgreement
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_CounterpartyNameAxis=mgnx_IMabBiopharmaMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |