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

Potential clinical milestone payment per IND acceptance under collaboration agreement.

+ References

No definition available.

+ Details

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

- 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 license grant fees available to be earned under collaboration agreement.

+ References

No definition available.

+ Details

Name:	mgnx_AdditionalPotentialLicenseGrantFeesUnderAgreement
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

Collaboration arrangement offset to research and development costs.

+ References

No definition available.

+ Details

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

- Definition

Collaborative arrangement expected development period.

+ References

No definition available.

+ Details

Name:	mgnx_CollaborativeArrangementExpectedDevelopmentPeriod
Namespace Prefix:	mgnx_
Data Type:	xbrli:durationItemType
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

Original recognition period for development.

+ References

No definition available.

+ Details

Name:	mgnx_OriginalPeriodOfDevelopment
Namespace Prefix:	mgnx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- 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

- Definition

Upfront payment recognition period.

+ References

No definition available.

+ Details

Name:	mgnx_UpfrontPaymentRecognitionPeriod
Namespace Prefix:	mgnx_
Data Type:	xbrli:durationItemType
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

Cash received from licensees for license fees during the current period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_ProceedsFromLicenseFeesReceived
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_LesLaboratoiresServierAndInstitutDeRecherchesServierMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and License Agreements, Les Laboratoires Servier (Details) (USD $)	1 Months Ended		3 Months Ended		1 Months Ended
	May 31, 2014	Nov. 30, 2011	Mar. 31, 2015	Mar. 31, 2014	Sep. 30, 2012	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
Servier [Member] \| Servier MGA271 [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date			November 2011
Non-refundable upfront payment		20,000,000
Additional license grant fees			30,000,000
Additional Potential Clinical Milestone Payments Under Agreement			47,000,000
Additional Potential Regulatory Milestone Payments Under Agreement			140,000,000
Additional Potential Sales Milestone Payments Under Agreement			208,000,000
Original period of development			27 months
Expected period of development			42 months
Recognized revenue under agreement			100,000	200,000
Milestone payment				0
Deferred revenue			36,000			100,000
Servier [Member] \| Servier DART [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date			September 2012
Non-refundable upfront payment					20,000,000
Additional license grant fees			65,000,000
Additional Potential Clinical Milestone Payments Under Agreement			98,000,000
Additional Potential Regulatory Milestone Payments Under Agreement			300,000,000
Additional Potential Sales Milestone Payments Under Agreement			630,000,000
Original period of development			29 months
Expected period of development			75 months
Recognized revenue under agreement			800,000	7,400,000
Milestone payment			0
Deferred revenue			16,900,000			17,700,000
License option fees received				15,000,000
Clinical milestone payments received under agreement				5,000,000
Offset to research and development costs under collaboration arrangement			300,000
Expected development period			82 months
Clinical milestone payments under agreement				5,000,000
Review period			30 days
Deferred revenue included in current liabilities			3,300,000			3,300,000
Deferred revenue included in long-term liabilities			$ 13,600,000			$ 14,400,000