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Collaboration and License Agreements - Additional Information 5 (Detail) (USD $)	1 Months Ended	12 Months Ended
	Oct. 31, 2010 Molecule	Dec. 31, 2014 Program	Dec. 31, 2013	Dec. 31, 2012
License And Collaboration Agreements [Line Items]
Deferred revenue		$ 14,248,000us-gaap_DeferredRevenueCurrent	$ 20,267,000us-gaap_DeferredRevenueCurrent
Pfizer [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date		October 2010
Period of agreement		3 years
Commercialization of molecules	2mgnx_NumberOfMoleculesToBeDevelopedAndCommercializedAsPerCollaborations / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Nonrefundable upfront payment	5,000,000mgnx_NonRefundableUpFrontPayment / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember	5,000,000mgnx_NonRefundableUpFrontPayment / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Clinical milestone payments		17,000,000mgnx_AdditionalPotentialClinicalMilestonePaymentsUnderAgreement / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Commercialization and sales milestones		195,000,000mgnx_AdditionalPotentialCommercializationAndSalesMilestonePaymentsUnderAgreement / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
DART program		1mgnx_NumberOfPrograms / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Research obligation completion date		2014-01
Term of the agreement		2014-01
Recognized revenue under agreement		100,000us-gaap_LicensesRevenue / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember	3,500,000us-gaap_LicensesRevenue / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember	5,500,000us-gaap_LicensesRevenue / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Milestones achieved		0us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember		500,000us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember
Deferred revenue		$ 0us-gaap_DeferredRevenueCurrent / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember	$ 7,000us-gaap_DeferredRevenueCurrent / us-gaap_CounterpartyNameAxis = mgnx_PfizerIncMember

X

- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

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- Definition

Collaboration and license agreement entered date.

+ References

No definition available.

+ Details

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- Definition

Collaborative agreement period of contract.

+ References

No definition available.

+ Details

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- Definition

Non-refundable up front payment.

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Number of programs being pursued under collaboration agreement.

+ References

No definition available.

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- Definition

Research obligation completion date.

+ References

No definition available.

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- Definition

Upfront fee and participation recognition end date.

+ References

No definition available.

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- 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



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section 45

-Paragraph 8

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-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

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- 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

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- 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

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