Additional funding available under agreement at counterparty's option

No definition available.

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

- Definition

Funded value of base period

+ References

No definition available.

+ Details

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

- Definition

Number of molecules to be developed and commercialized as per collaboration.

+ References

No definition available.

+ Details

Name:	mgnx_NumberOfMoleculesToBeDevelopedAndCommercializedUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to total potential value under agreement.

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue earned during the period relating to consideration received from another party for the right to use, but not own, certain of the entity's intangible assets. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. Licensing fees are generally, but not always, fixed as to amount and not dependent upon the revenue generated by the licensing party. An entity may receive licensing fees for licenses that also generate royalty payments to the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

+ Details

Name:	us-gaap_LicensesRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_CounterpartyNameAxis=mgnx_NationalInstituteOfAllergyAndInfectiousDiseasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and Other Agreements, NIAID Contract (Details) - NIAID		12 Months Ended
	Sep. 15, 2015 USD ($) Molecule	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaboration And Other Agreements [Line Items]
Commercialization of molecules \| Molecule	2
Base period value	$ 7,500,000
Recognized revenue under agreement		$ 5,100,000	$ 200,000
Maximum
Collaboration And Other Agreements [Line Items]
Additional development funding options	17,000,000.0
Total potential value	$ 24,500,000.0