XML 56 R43.htm IDEA: XBRL DOCUMENT v2.4.1.9
Income Taxes - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Line Items]      
Provision for federal or state income taxes $ 0us-gaap_CurrentIncomeTaxExpenseBenefit $ 0us-gaap_CurrentIncomeTaxExpenseBenefit $ 0us-gaap_CurrentIncomeTaxExpenseBenefit
Federal Tax credit expiration start year 2020    
Federal Tax credit expiration ending year 2034    
Net operating loss 7,100,000mgnx_OperatingLossCarryforwardsLimitationsOnUseAmount    
Net operating losses limited for year minimum 2015    
Net operating losses limited for year maximum 2027    
Remaining portion of net operating losses 151,800,000mgnx_OperatingLossCarryforwardsNotSubjectToLimitation    
Net operating losses, recognized as a benefit through additional-paid-in-capital 7,700,000mgnx_UnrecognizedTaxBenefitsResultingInNetOperatingLossesCarryforward    
Current deferred tax liability 1,600,000us-gaap_DeferredTaxLiabilitiesGrossCurrent    
Noncurrent deferred tax asset 1,600,000us-gaap_DeferredTaxAssetsGrossNoncurrent    
Net deferred income tax asset/(liability) 0us-gaap_DeferredTaxAssetsLiabilitiesNet 0us-gaap_DeferredTaxAssetsLiabilitiesNet  
Gross unrecognized tax benefits 1,700,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate 1,300,000us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate  
Unrecognized interest or penalties 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense 0us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense
Income tax examination year subject to examination, Range start 2001    
Income tax examination year subject to examination, Range End 2014    
Minimum [Member]      
Income Tax Disclosure [Line Items]      
Net operating losses limited for use utilized on an annual basis 200,000mgnx_OperatingLossCarryforwardsAnnualLimitationAmount
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum [Member]      
Income Tax Disclosure [Line Items]      
Net operating losses limited for use utilized on an annual basis 1,400,000mgnx_OperatingLossCarryforwardsAnnualLimitationAmount
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
U.S. Federal Government [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards 158,900,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
US Federal Tax credits carry forward 26,300,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_DomesticCountryMember		    
State and Local Jurisdiction [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards $ 103,400,000us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember		    
Start Year [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards expiration ending year 2020    
Ending Year [Member]      
Income Tax Disclosure [Line Items]      
Net operating loss carryforwards expiration ending year 2034