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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