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Collaboration and License Agreements - Additional Information 6 (Detail) (USD $)	1 Months Ended	12 Months Ended
	Jun. 30, 2010	Dec. 31, 2014	Dec. 31, 2013
License And Collaboration Agreements [Line Items]
Deferred revenue, Current		$ 14,248,000us-gaap_DeferredRevenueCurrent	$ 20,267,000us-gaap_DeferredRevenueCurrent
Deferred revenue, Long-term liabilities		16,472,000us-gaap_DeferredRevenueNoncurrent	7,136,000us-gaap_DeferredRevenueNoncurrent
Green Cross [Member]
License And Collaboration Agreements [Line Items]
Collaboration agreement date		June 2010
Term of the agreement		2020-06
Nonrefundable upfront payment	1,000,000mgnx_NonRefundableUpFrontPayment / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Adjustment to revenue from contract material modification		1,300,000mgnx_AdjustmentToRevenueUnderAccountingStandardsCodification / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Recognized revenue under agreement		1,700,000us-gaap_LicensesRevenue / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember	100,000.0us-gaap_LicensesRevenue / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Milestones achieved		0us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember	0us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Deferred revenue			650,000us-gaap_DeferredRevenue / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Unbilled receivable balance		525,000us-gaap_UnbilledContractsReceivable / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Deferred revenue, Current			100,000us-gaap_DeferredRevenueCurrent / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Deferred revenue, Long-term liabilities			550,000us-gaap_DeferredRevenueNoncurrent / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember
Green Cross [Member] | Maximum [Member]
License And Collaboration Agreements [Line Items]
Aggregate potential future cost reimbursement		5,500,000mgnx_AggregatePotentialFutureCostReimbursement / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember / us-gaap_RangeAxis = us-gaap_MaximumMember
Clinical and commercial milestone payments		$ 2,500,000mgnx_AdditionalPotentialClinicalAndCommercialMilestonePaymentsUnderAgreement / us-gaap_CounterpartyNameAxis = mgnx_GreenCrossCorporationMember / us-gaap_RangeAxis = us-gaap_MaximumMember

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- Definition

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

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Collaboration and license agreement entered date.

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Non-refundable up front payment.

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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.

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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.

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