Collaboration and License Agreements, Takeda Pharmaceutical Company Limited (Details) (USD $)	1 Months Ended	3 Months Ended
May 31, 2014	Mar. 31, 2015	Dec. 31, 2014
License And Collaboration Agreements [Line Items]
Non-refundable upfront payment	$ 15,000,000mgnx_NonRefundableUpfrontFees
Deferred revenue current		12,382,000us-gaap_DeferredRevenueCurrent	14,248,000us-gaap_DeferredRevenueCurrent
Deferred revenue non-current		14,389,000us-gaap_DeferredRevenueNoncurrent	16,472,000us-gaap_DeferredRevenueNoncurrent
Takeda Pharmaceutical Company Limited [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date		May 2014
License option fee		15,000,000
Upfront fee allocated to agreement		10,000,000
Expected period of development		24 months
Recognized revenue under agreement		4,300,000
Deferred revenue		5,800,000	7,100,000
Deferred revenue current		5,000,000	5,000,000
Deferred revenue non-current		800,000	2,100,000
Revenue Recognition, Milestone Method, Revenue Recognized		3,000,000
Additional Potential Sales Milestone Payments Under Agreement		375,500,000
Additional Potential Clinical And Regulatory Milestone Payments Under Agreement		93,000,000
Takeda Pharmaceutical Company Limited [Member] \| Research Collaboration And License Option Agreement [Member]
License And Collaboration Agreements [Line Items]
Upfront fee allocated to agreement		5,000,000
Research and development services revenue		$ 300,000

Additional potential clinical and regulatory payments under agreement.

No definition available.

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- Definition

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

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

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- Definition

Initial fee received from collaboration or license agreement.

+ References

No definition available.

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- Definition

Potential license option fee.

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No definition available.

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- Definition

Revenue recognized from reimbursement of research and development services.

+ References

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- Definition

Portion of upfront fee allocated to agreement.

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No definition available.

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Upfront payment recognition period.

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- 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

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-Topic 605

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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.

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

-Publisher FASB

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-Topic 210

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- 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.

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

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 28

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