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Income Taxes - Reconciliation of Reported Estimated Income Tax Benefit (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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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 | $ 2,927 us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
State taxes (net of federal benefit) | (1,608)us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | 609 us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes | 1,460 us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes
Deferred income tax adjustments |  | (855)mgnx_IncomeTaxReconciliationDeferredTaxAdjustments | (512)mgnx_IncomeTaxReconciliationDeferredTaxAdjustments
Deferred state blended rate adjustments | 3,034 mgnx_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 | 3 us-gaap_IncomeTaxReconciliationOtherReconcilingItems | 8 us-gaap_IncomeTaxReconciliationOtherReconcilingItems
Equity-based compensation | 756 us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost | 241 us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost | 279 us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Fair value adjustment of preferred stock warrant liability |  | 219 mgnx_IncomeTaxReconciliationFairValueAdjustmentOfPreferredStockWarrantLiability | (53)mgnx_IncomeTaxReconciliationFairValueAdjustmentOfPreferredStockWarrantLiability
Change in valuation allowance | 14,920 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 1,287 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance | 787 us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance
Income tax expense/(benefit) | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-gaap_IncomeTaxExpenseBenefit | 0 us-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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- Definition Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(h)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Income Tax Expense (or Benefit) -URI http://asc.fasb.org/extlink&oid=6515339 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32639-109319 + Details
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- Definition 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 increase (decrease) in the valuation allowance for deferred tax assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32123-109318 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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- Definition The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Subparagraph 2 -Article 4 + Details
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- Definition 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 nondeductible equity-based compensation costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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- Definition 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 tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 + Details
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