Additional clinical milestone payment received under agreement.

No definition available.

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

- Definition

Aggregate potential clinical milestone payments available to be earned under collaboration agreement.

+ References

No definition available.

+ Details

Name:	mgnx_AdditionalPotentialClinicalMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Aggregate potential regulatory milestone payments available to be earned under a collaboration arrangement.

+ References

No definition available.

+ Details

Name:	mgnx_AdditionalPotentialRegulatoryMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Aggregate potential sales milestone payments available to be earned under collaboration agreement.

+ References

No definition available.

+ Details

Name:	mgnx_AdditionalPotentialSalesMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Date on which the Company and collaborator entered into collaboration agreement.

+ References

No definition available.

+ Details

Name:	mgnx_CollaborationAndLicenseAgreementEnteredDate
Namespace Prefix:	mgnx_
Data Type:	invest:dateStringItemType
Balance Type:	na
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

Number of annual maintenance payment received.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Research obligation completion date.

+ References

No definition available.

+ Details

Name:	mgnx_ResearchObligationCompletionDate
Namespace Prefix:	mgnx_
Data Type:	invest:dateStringItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Definition

Amount of deferred revenue as of balance sheet date. Deferred revenue represents collections of cash or other assets related to a revenue producing activity for which revenue has not yet been recognized. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 13

-Section A

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 13.A.4(a).Q1 Response)

-URI http://asc.fasb.org/extlink&oid=27012821&loc=d3e214044-122780

+ Details

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

- Definition

The carrying amount of consideration received or receivable as of the balance sheet date on potential earnings that were not recognized as revenue in conformity with GAAP, and which are expected to be recognized as such within one year or the normal operating cycle, if longer, including sales, license fees, and royalties, but excluding interest income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 13.A.4(a).Q1)

-URI http://asc.fasb.org/extlink&oid=27012821&loc=d3e214044-122780

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section 45

-Paragraph 8

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6935-107765

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 13

-Section A

+ Details

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

- Definition

The noncurrent portion of deferred revenue amount as of balance sheet date. Deferred revenue is a liability related to a revenue producing activity for which revenue has not yet been recognized, and is not expected to be recognized in the next twelve months. Generally, an entity records deferred revenue when it receives consideration from a customer before achieving certain criteria that must be met for revenue to be recognized in conformity with GAAP.

+ References

+ Details

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

- 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

- Definition

The amount of consideration recognized during the period for the milestone or milestones.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 28

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=19771982&loc=SL6892177-166501

+ Details

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

- Details

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

Collaboration and License Agreements, Boehringer Ingelheim International GmbH (Details) (USD $)	1 Months Ended		3 Months Ended
	May 31, 2014	Oct. 31, 2010 Molecule	Mar. 31, 2015 Payment	Mar. 31, 2014	Dec. 31, 2014
License And Collaboration Agreements [Line Items]
Non-refundable upfront payment	$ 15,000,000mgnx_NonRefundableUpfrontFees
Deferred revenue included in current liabilities			12,382,000us-gaap_DeferredRevenueCurrent		14,248,000us-gaap_DeferredRevenueCurrent
Deferred revenue included in long-term liabilities			14,389,000us-gaap_DeferredRevenueNoncurrent		16,472,000us-gaap_DeferredRevenueNoncurrent
Boehringer [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date			October 2010
Commercialization of molecules		10
Non-refundable upfront payment		15,000,000
Number of annual maintenance payments received			3
Additional Potential Clinical Milestone Payments Under Agreement			41,000,000
Additional Potential Regulatory Milestone Payments Under Agreement			89,000,000
Additional Potential Sales Milestone Payments Under Agreement			83,000,000
Research obligation completion date			September 2015
Clinical milestone payments under agreement
Recognized revenue under agreement			2,600,000	3,100,000
Deferred revenue			4,000,000		5,800,000
Deferred revenue included in current liabilities			4,000,000		5,800,000
Deferred revenue included in long-term liabilities			0		0
Milestone payment