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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Reconciliation [Line Items]
United States federal tax at statutory rate	$ (13,410)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ (91)us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 2,927us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State taxes (net of federal benefit)	(1,608)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	609us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes	1,460us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Deferred income tax adjustments		(855)mgnx_IncomeTaxReconciliationDeferredTaxAdjustments	(512)mgnx_IncomeTaxReconciliationDeferredTaxAdjustments
Deferred state blended rate adjustments	3,034mgnx_IncomeTaxReconciliationStateDeferredTaxesAdjustments	(344)mgnx_IncomeTaxReconciliationStateDeferredTaxesAdjustments
Research credit, net	(2,228)us-gaap_IncomeTaxReconciliationTaxCreditsResearch	(226)us-gaap_IncomeTaxReconciliationTaxCreditsResearch
Transaction cost deduction	(379)us-gaap_IncomeTaxReconciliationDeductions
Orphan drug credit, net	(139)us-gaap_IncomeTaxReconciliationTaxCreditsOther	(843)us-gaap_IncomeTaxReconciliationTaxCreditsOther	(4,896)us-gaap_IncomeTaxReconciliationTaxCreditsOther
Other permanent items	(382)us-gaap_IncomeTaxReconciliationOtherReconcilingItems	3us-gaap_IncomeTaxReconciliationOtherReconcilingItems	8us-gaap_IncomeTaxReconciliationOtherReconcilingItems
Equity-based compensation	756us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	241us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	279us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Fair value adjustment of preferred stock warrant liability		219mgnx_IncomeTaxReconciliationFairValueAdjustmentOfPreferredStockWarrantLiability	(53)mgnx_IncomeTaxReconciliationFairValueAdjustmentOfPreferredStockWarrantLiability
Change in valuation allowance	14,920us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	1,287us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance	787us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Income tax expense/(benefit)	0us-gaap_IncomeTaxExpenseBenefit	0us-gaap_IncomeTaxExpenseBenefit	0us-gaap_IncomeTaxExpenseBenefit
Prior Year Adjustment [Member]
Effective Income Tax Rate Reconciliation [Line Items]
Transaction cost deduction	$ (564)us-gaap_IncomeTaxReconciliationDeductions / us-gaap_StatementScenarioAxis = us-gaap_ScenarioPreviouslyReportedMember

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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