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Collaboration and License Agreements - Additional Information 1 (Detail) (USD $)
1 Months Ended 12 Months Ended 1 Months Ended
Feb. 28, 2014
Dec. 31, 2014
Dec. 31, 2013
May 31, 2014
License And Collaboration Agreements [Line Items]        
Stock issued during period value, new issues   $ 76,716,000us-gaap_StockIssuedDuringPeriodValueNewIssues $ 83,623,000us-gaap_StockIssuedDuringPeriodValueNewIssues  
Stock issued during the period for new issue 1,800,000us-gaap_StockIssuedDuringPeriodSharesNewIssues      
Deferred revenue current   14,248,000us-gaap_DeferredRevenueCurrent 20,267,000us-gaap_DeferredRevenueCurrent  
Deferred revenue non-current   16,472,000us-gaap_DeferredRevenueNoncurrent 7,136,000us-gaap_DeferredRevenueNoncurrent  
Janssen Biotech Inc. [Member]        
License And Collaboration Agreements [Line Items]        
Collaboration agreement date   December 2014    
Stock issued during period value, new issues   75,000,000us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Stock issued during the period for new issue   1,923,077us-gaap_StockIssuedDuringPeriodSharesNewIssues
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Common stock price per share   $ 39.00us-gaap_SaleOfStockPricePerShare
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Nonrefundable upfront payment receivable   50,000,000mgnx_NonRefundableUpfrontPaymentReceivable
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Clinical, regulatory and commercialization milestone payments   575,000,000mgnx_AdditionalPotentialClinicalRegulatoryCommercializationPaymentsUnderAgreement
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Recognized revenue under agreement   0us-gaap_LicensesRevenue
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Deferred revenue   0us-gaap_DeferredRevenue
/ us-gaap_CounterpartyNameAxis
= mgnx_JanssenBiotechIncMember
   
Takeda [Member]        
License And Collaboration Agreements [Line Items]        
Collaboration agreement date   May 2014    
Recognized revenue under agreement   3,000,000us-gaap_LicensesRevenue
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Deferred revenue   7,100,000us-gaap_DeferredRevenue
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Nonrefundable upfront payment       15,000,000mgnx_NonRefundableUpFrontPayment
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
License option fee and early development milestone   18,000,000mgnx_PotentialLicenseOptionFeeAndEarlyDevelopmentMilestoneFee
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Development, regulatory and sales milestone payments   468,500,000mgnx_AdditionalPotentialDevelopmentRegulatoryAndSalesMilestoneUnderAgreement
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Amount allocated to agreement   10,000,000mgnx_UpfrontFeeAllocatedToAgreement
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Expected period of development   24 months    
Deferred revenue current   5,000,000us-gaap_DeferredRevenueCurrent
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Deferred revenue non-current   2,100,000us-gaap_DeferredRevenueNoncurrent
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
   
Takeda [Member] | Research Collaboration And License Option Agreement [Member]        
License And Collaboration Agreements [Line Items]        
Collaboration agreement date   September 2014    
Recognized revenue under agreement   5,000,000us-gaap_LicensesRevenue
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
/ us-gaap_TypeOfArrangementAxis
= mgnx_ResearchCollaborationAndLicenseOptionAgreementMember
   
Amount allocated to agreement   $ 5,000,000mgnx_UpfrontFeeAllocatedToAgreement
/ us-gaap_CounterpartyNameAxis
= mgnx_TakedaMember
/ us-gaap_TypeOfArrangementAxis
= mgnx_ResearchCollaborationAndLicenseOptionAgreementMember