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Income Taxes - Valuation Allowance Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Valuation Allowance [Roll Forward]
Balance at Beginning of Year	$ 372,267	$ 327,592	$ 263,400
Additions	0	44,675	64,192
Deductions	(7,257)	0	0
Balance at End of Year	$ 365,010	$ 372,267	$ 327,592

X

- Definition

Valuation Allowance, Deferred Tax Asset, Decrease, Amount

+ References

No definition available.

+ Details

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

X

- Definition

Valuation Allowance, Deferred Tax Asset, Increase, Amount

+ References

No definition available.

+ Details

Name:	mgnx_ValuationAllowanceDeferredTaxAssetIncreaseAmount
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Valuation Allowance

+ References

No definition available.

+ Details

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

X

- Definition

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-2

+ Details

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant