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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

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- Definition

Future Revenue

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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]

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No definition available.

+ Details

Name:	mgnx_LiabilityRelatedToFutureRoyaltyLineItems
Namespace Prefix:	mgnx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Net proceeds from sale of future royalties

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No definition available.

+ Details

Name:	mgnx_NetProceedsFromSaleOfFutureRoyalties
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

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Name:	mgnx_ProceedsfromRoyaltiesPercent
Namespace Prefix:	mgnx_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

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- 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

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- Details

Name:	srt_CounterpartyNameAxis=mgnx_DRIHealthcareAcquisitionsLPDRIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: