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Lease Exit Liability - Additional Information (Detail) (USD $)
 12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 16, 2014
Jul. 16, 2008
Restructuring Cost and Reserve [Line Items]          
Operating Lease 2018        
Onetime purchase payment under purchase agreement         $ 5,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreement
Additional amounts recorded 0mgnx_ContingentConsiderationRecordedAsIncrementalInProcessResearchAndDevelopmentExpense 0mgnx_ContingentConsiderationRecordedAsIncrementalInProcessResearchAndDevelopmentExpense 0mgnx_ContingentConsiderationRecordedAsIncrementalInProcessResearchAndDevelopmentExpense    
Minimum [Member]          
Restructuring Cost and Reserve [Line Items]          
Onetime payment paid under license of product         8,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreementUponSpecifiedLevelOfSalesOfProducts
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum [Member]          
Restructuring Cost and Reserve [Line Items]          
Onetime payment paid under license of product         $ 12,000,000mgnx_OnetimeMilestonePaymentToBePaidUnderAgreementUponSpecifiedLevelOfSalesOfProducts
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
Series D Convertible Preferred Stock [Member]          
Restructuring Cost and Reserve [Line Items]          
Shares issued in connection with acquisition       12,466,039us-gaap_StockIssuedDuringPeriodSharesAcquisitions
/ us-gaap_StatementClassOfStockAxis
= mgnx_ConvertiblePreferredStockSeriesDMember