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Lease Exit Liability (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Jul. 16, 2008
Restructuring Cost and Reserve [Line Items]      
Lease expiration year 2018    
Restructuring Costs [Abstract]      
Onetime purchase payment under purchase agreement     $ 5,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreement
Incremental in-process research and development expense 0mgnx_ContingentConsiderationRecordedAsIncrementalInProcessResearchAndDevelopmentExpense 0mgnx_ContingentConsiderationRecordedAsIncrementalInProcessResearchAndDevelopmentExpense  
Minimum [Member]      
Restructuring Costs [Abstract]      
Onetime payment paid under license of product     8,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreementUponSpecifiedLevelOfSalesOfProducts
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]      
Restructuring Costs [Abstract]      
Onetime payment paid under license of product     12,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreementUponSpecifiedLevelOfSalesOfProducts
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Contract Termination [Member]      
Restructuring Costs [Abstract]      
Accrual beginning balance 8,006,000us-gaap_RestructuringReserve
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_ContractTerminationMember
   
Principal payments (396,000)us-gaap_PaymentsForRestructuring
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_ContractTerminationMember
   
Accrual ending balance $ 7,610,000us-gaap_RestructuringReserve
/ us-gaap_RestructuringCostAndReserveAxis
= us-gaap_ContractTerminationMember