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

No definition available.

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

- Definition

Represents the adjustment to revenue under the accounting standards codification.

+ References

No definition available.

+ Details

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

- Definition

Aggregate potential future cost reimbursement from collaborator

+ References

No definition available.

+ Details

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

- Definition

Date on which the Company entered into agreement.

+ References

No definition available.

+ Details

Name:	mgnx_CollaborationAndOtherAgreementEnteredDate
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

Upfront fee and participation recognition end date.

+ References

No definition available.

+ Details

Name:	mgnx_UpfrontFeeAndParticipationRecognitionEndDate
Namespace Prefix:	mgnx_
Data Type:	invest:dateStringItemType
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

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 210

-SubTopic 10

-Section 45

-Paragraph 8

-Subparagraph (b)

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

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)

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

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

Unbilled amounts due for services rendered or to be rendered, actions taken or to be taken, or a promise to refrain from taking certain actions in accordance with the terms of a legally binding agreement between the entity and, at a minimum, one other party. An example would be amounts associated with contracts or programs where the recognized revenue for performance thereunder exceeds the amounts billed under the terms thereof as of the date of the balance sheet.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3(c)(3))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 912

-SubTopic 310

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=51666815&loc=d3e55302-109406

+ Details

Name:	us-gaap_UnbilledContractsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

Collaboration and Other Agreements, Green Cross Corporation (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2010	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Collaboration And Other Agreements [Line Items]
Deferred revenue included in current liabilities		$ 7,163		$ 7,163		$ 14,248
Deferred revenue included in long-term liabilities		11,889		$ 11,889		16,472
Green Cross [Member]
Collaboration And Other Agreements [Line Items]
Agreement beginning date				June 2010
Aggregate potential future cost reimbursement		5,500		$ 5,500
Additional Potential Clinical and Commercial Milestone Payments Under Agreement		2,500		$ 2,500
Term of the agreement				June 2020
Non-refundable upfront payment	$ 1,000
Adjustment to revenue					$ 1,300
Recognized revenue under agreement		200	$ 100	$ 400	$ 1,600
Unbilled receivable balance		$ 900		$ 900		$ 500