Collaborative Agreement Transaction Price

No definition available.

Name:	mgnx_CollaborativeAgreementTransactionPrice
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Development And Regulatory Milestone Payments Under Agreement, Recognized

+ References

No definition available.

+ Details

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

- Definition

Initial fee received from collaboration or license agreement.

+ References

No definition available.

+ Details

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

- Definition

Potential Commercial Milestone Payments Under Agreement

+ References

No definition available.

+ Details

Name:	mgnx_PotentialCommercialMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Potential Development and Regulatory Milestone Payments Under Agreement

+ References

No definition available.

+ Details

Name:	mgnx_PotentialDevelopmentandRegulatoryMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Proceeds from Royalties, Percent

+ References

No definition available.

+ Details

Name:	mgnx_ProceedsfromRoyaltiesPercent
Namespace Prefix:	mgnx_
Data Type:	num:percentItemType
Balance Type:	na
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

Represents the value established as the selling price of the significant unit of accounting used to allocate the consideration of the multiple-deliverable arrangement.

+ References

No definition available.

+ Details

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

- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=115205541&loc=SL114868664-224227

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

+ Details

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

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=mgnx_IncyteMGA012AgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=mgnx_RevenuesFromLicenseAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=mgnx_IncyteMGA012ClinicalServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and Other Agreements, Incyte Corporation (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Collaboration and Other Agreements [Line Items]
Revenues		$ 15,794	$ 20,798	$ 18,834	$ 4,695	$ 152,359	$ 1,663	$ 1,666	$ 2,055	$ 60,121	$ 157,742	$ 91,880
Incyte MGA012 Agreement \| Incyte Corporation (Incyte)
Collaboration and Other Agreements [Line Items]
Non-refundable upfront payment						$ 150,000
Amounts recognized										15,000
Transaction price	$ 154,000
Clinical trial activities selling price amount	4,000
Maximum \| Incyte MGA012 Agreement \| Incyte Corporation (Incyte)
Collaboration and Other Agreements [Line Items]
Potential development and regulatory milestone payments	420,000
Potential commercial milestone payments	$ 330,000
Potential proceeds from royalties percent	24.00%
Minimum \| Incyte MGA012 Agreement \| Incyte Corporation (Incyte)
Collaboration and Other Agreements [Line Items]
Potential proceeds from royalties percent	15.00%
Revenues From License Agreements \| Incyte MGA012 Agreement \| Incyte Corporation (Incyte)
Collaboration and Other Agreements [Line Items]
Revenues										18,800	$ 151,100
Revenues From License Agreements \| Incyte MGA012 Clinical Services \| Incyte Corporation (Incyte)
Collaboration and Other Agreements [Line Items]
Revenues										$ 22,200