Additional Clinical Milestone Potential Payments Under Agreement

No definition available.

Name:	mgnx_AdditionalClinicalMilestonePotentialPaymentsUnderAgreement
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

Additional potential milestone payments receivable upon new drug application acceptance.

+ References

No definition available.

+ Details

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

- Definition

Aggregate potential remaining 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 remaining 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

Disclosure of the effect of a change in accounting estimate on current year operations.

+ References

No definition available.

+ Details

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

- Definition

Collaboration Agreement Consideration Receivable

+ References

No definition available.

+ Details

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

- Definition

Collaboration and license agreement entered date.

+ References

No definition available.

+ Details

Name:	mgnx_CollaborationAndLicenseAgreementEnteredDate
Namespace Prefix:	mgnx_
Data Type:	us-types: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

- Details

Name:	mgnx_LicenseAndCollaborationAgreementsLineItems
Namespace Prefix:	mgnx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Non-refundable up front payment.

+ References

No definition available.

+ Details

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

- Definition

Number of clinical milestones achieved during period.

+ References

No definition available.

+ Details

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

- Definition

Payment from collaborator upon exercise of option to develop and commercialize product.

+ References

No definition available.

+ Details

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

- Definition

Option exercise fee recognition period.

+ References

No definition available.

+ Details

Name:	mgnx_OptionExerciseFeeRecognitionPeriod
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

Period over which upfront payment is to be recognized.

+ References

No definition available.

+ Details

Name:	mgnx_UpfrontPaymentRecognitionPeriod
Namespace Prefix:	mgnx_
Data Type:	xbrli:durationItemType
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_ServierMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and License Agreements - Additional Information 3 (Detail) (USD $)	12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended
	Dec. 31, 2014	Nov. 30, 2011	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2012
License And Collaboration Agreements [Line Items]
Deferred revenue, Current	$ 14,248,000us-gaap_DeferredRevenueCurrent		$ 20,267,000us-gaap_DeferredRevenueCurrent
Change in accounting estimate, current year impact	3,700,000
Deferred revenue, Long-term liabilities	16,472,000us-gaap_DeferredRevenueNoncurrent		7,136,000us-gaap_DeferredRevenueNoncurrent
Servier [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date	November 2011
Nonrefundable upfront payment		20,000,000
Expected period of development	27 months
Recognized revenue under agreement	700,000		21,300,000	9,100,000
Milestone payment			10,000,000
Deferred revenue, Current	100,000		900,000
Servier [Member] \| Second Right to Develop Collaboration [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date	September 2012
Nonrefundable upfront payment					20,000,000
Expected period of development	29 months
Recognized revenue under agreement	16,700,000		8,600,000	2,000,000
Milestone payment	5,000,000		0	0
Deferred revenue, Current	3,300,000		8,600,000
Option exercise fee	15,000,000
Offset to research and development costs under collaboration arrangement	1,000,000		0	0
Option exercise fee recognition period	82 months
Collaboration receivable, included in accounts receivable	800,000
Number of milestones achieved	2
Review period	30 days
Deferred revenue	17,700,000		9,400,000
Deferred revenue, Long-term liabilities	14,400,000		800,000
Servier [Member] \| Scenario, Adjustment [Member]
License And Collaboration Agreements [Line Items]
Expected period of development	42 months
Servier [Member] \| Scenario, Adjustment [Member] \| Second Right to Develop Collaboration [Member]
License And Collaboration Agreements [Line Items]
Expected period of development	75 months
Servier [Member] \| Maximum [Member]
License And Collaboration Agreements [Line Items]
Additional license grant fees	30,000,000
Clinical milestone payments	47,000,000
Regulatory milestone payments	140,000,000
Sales milestone payments	208,000,000
Servier [Member] \| Maximum [Member] \| Second Right to Develop Collaboration [Member]
License And Collaboration Agreements [Line Items]
Additional license grant fees	65,000,000
Clinical milestone payments	98,000,000
Regulatory milestone payments	300,000,000
Sales milestone payments	630,000,000
Clinical milestone payments under agreement	$ 5,000,000