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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 | $ 0 us-gaap_CurrentIncomeTaxExpenseBenefit | $ 0 us-gaap_CurrentIncomeTaxExpenseBenefit | $ 0 us-gaap_CurrentIncomeTaxExpenseBenefit
Federal Tax credit expiration start year | 2020 |  |
Federal Tax credit expiration ending year | 2034 |  |
Net operating loss | 7,100,000 mgnx_OperatingLossCarryforwardsLimitationsOnUseAmount |  |
Net operating losses limited for year minimum | 2015 |  |
Net operating losses limited for year maximum | 2027 |  |
Remaining portion of net operating losses | 151,800,000 mgnx_OperatingLossCarryforwardsNotSubjectToLimitation |  |
Net operating losses, recognized as a benefit through additional-paid-in-capital | 7,700,000 mgnx_UnrecognizedTaxBenefitsResultingInNetOperatingLossesCarryforward |  |
Current deferred tax liability | 1,600,000 us-gaap_DeferredTaxLiabilitiesGrossCurrent |  |
Noncurrent deferred tax asset | 1,600,000 us-gaap_DeferredTaxAssetsGrossNoncurrent |  |
Net deferred income tax asset/(liability) | 0 us-gaap_DeferredTaxAssetsLiabilitiesNet | 0 us-gaap_DeferredTaxAssetsLiabilitiesNet |
Gross unrecognized tax benefits | 1,700,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | 1,300,000 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate |
Unrecognized interest or penalties | 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 0 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestExpense | 0 us-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 | 200,000 mgnx_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 | 1,400,000 mgnx_OperatingLossCarryforwardsAnnualLimitationAmount / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
U.S. Federal Government [Member] |  |  |
Income Tax Disclosure [Line Items] |  |  |
Net operating loss carryforwards | 158,900,000 us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsSubjectToExpiration / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |  |
US Federal Tax credits carry forward | 26,300,000 us-gaap_TaxCreditCarryforwardAmount / us-gaap_IncomeTaxAuthorityAxis = us-gaap_DomesticCountryMember |  |
State and Local Jurisdiction [Member] |  |  |
Income Tax Disclosure [Line Items] |  |  |
Net operating loss carryforwards | $ 103,400,000 us-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 |  |

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