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Royalty Monetization Arrangement - Narrative (Details) - USD ($) $ in Thousands |  | 9 Months Ended
--- | --- | ---
Mar. 08, 2023 | Sep. 30, 2023 | Sep. 30, 2022
Liability related to future royalty [Line Items] |  |  |
Net proceeds from sale of future royalties |  | $ 149,655 | $ 0
DRI Healthcare Acquisitions LP (DRI) |  |  |
Liability related to future royalty [Line Items] |  |  |
Upfront Payment | $ 100,000 |  |
Transaction Cost | $ 300 |  |
Potential proceeds from royalties (percent) | 50.00% |  |
Future Revenue | $ 50,000 |  |

X
- Definition Future Revenue + References No definition available. + Details
Name: | mgnx_FutureRevenue
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Liability related to future royalty [Line Items] + References No definition available. + Details
Name: | mgnx_LiabilityRelatedToFutureRoyaltyLineItems
Namespace Prefix: | mgnx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Net proceeds from sale of future royalties + References No definition available. + Details
Name: | mgnx_NetProceedsFromSaleOfFutureRoyalties
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | mgnx_ProceedsfromRoyaltiesPercent
Namespace Prefix: | mgnx_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Transaction Cost + References No definition available. + Details
Name: | mgnx_TransactionCost
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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