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Stockholders' Equity - Additional Information (Detail) (USD $) | 0 Months Ended | 1 Months Ended | 12 Months Ended | 0 Months Ended | 24 Months Ended | 1 Months Ended | 0 Months Ended | 1 Months Ended |  |
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Oct. 16, 2013 | Feb. 28, 2014 | Dec. 31, 2014 | Jan. 11, 2011 | Dec. 31, 2003 | Oct. 31, 2004 | May 31, 2006 | Mar. 31, 2011 | Jul. 31, 2008 | May 16, 2010 | Sep. 30, 2008 | Dec. 31, 2013 | Dec. 31, 2012
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  | 1,800,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues |  |  |  |  |  |  |  |  |  |  |
Price per share |  | $ 36.50 us-gaap_SharePrice |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of shares |  | $ 76,700,000 us-gaap_ProceedsFromIssuanceOrSaleOfEquity |  |  |  |  |  |  |  |  |  |  |
Proceeds from initial public offering net of expenses | 83,800,000 us-gaap_ProceedsFromIssuanceInitialPublicOffering |  |  |  |  |  |  |  |  |  |  |  |
Reverse stock split |  |  | 1-for-18.7739 |  |  |  |  |  |  |  |  |  |
Common stock, shares authorized |  |  | 125,000,000 us-gaap_CommonStockSharesAuthorized |  |  |  |  |  |  |  |  | 125,000,000 us-gaap_CommonStockSharesAuthorized | 425,000,000 us-gaap_CommonStockSharesAuthorized
Additional shares sold under over-allotments option to underwriter |  | 450,000 mgnx_StockIssuedDuringPeriodSharesToUnderwritersForExerciseOfOverAllotmentOption |  |  |  |  |  |  |  |  |  |  |
IPO [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold | 5,000,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_SubsidiarySaleOfStockAxis = us-gaap_IPOMember |  |  |  |  |  |  |  |  |  |  |  |
Price per share | $ 16.00 us-gaap_SharePrice / us-gaap_SubsidiarySaleOfStockAxis = us-gaap_IPOMember |  |  |  |  |  |  |  |  |  |  |  |
Over-Allotment Option [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold | 750,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_SubsidiarySaleOfStockAxis = us-gaap_OverAllotmentOptionMember |  |  |  |  |  |  |  |  |  |  |  |
Price per share | $ 16.00 us-gaap_SharePrice / us-gaap_SubsidiarySaleOfStockAxis = us-gaap_OverAllotmentOptionMember |  |  |  |  |  |  |  |  |  |  |  |
Series D-2 Convertible Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  |  |  | 18,427,388 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = mgnx_SeriesD2ConvertiblePreferredStockMember |  |  |  |  |  |  |  |  |
Proceeds from sale of shares |  |  |  | 12,000,000 us-gaap_ProceedsFromIssuanceOrSaleOfEquity / us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis = mgnx_SeriesD2ConvertiblePreferredStockMember |  |  |  |  |  |  |  |  |
Series A Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  |  |  |  | 34,239,374 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesAPreferredStockMember |  |  |  |  |  |  |  |
Proceeds from issuance of convertible preferred stock |  |  |  |  | 34,000,000 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesAPreferredStockMember |  |  |  |  |  |  |  |
Price per share |  |  |  |  | $ 1.00 us-gaap_SharePrice / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesAPreferredStockMember |  |  |  |  |  |  |  |
Accrued noncumulative dividends on preferred stock |  |  | 0.08 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesAPreferredStockMember |  |  |  |  |  |  |  |  |  |
Series A-1 Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.506 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = mgnx_SeriesA1PreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 12.39 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = mgnx_SeriesA1PreferredStockMember |  |  |  |  |  |  |  |  |  |
Series A-2 Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.00 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = mgnx_SeriesA2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 18.77 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = mgnx_SeriesA2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Series B Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  |  |  |  |  | 71,401,237 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |
Proceeds from issuance of convertible preferred stock |  |  |  |  |  | 30,300,000 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |
Related offering costs |  |  |  |  |  | 238,000 mgnx_StockIssuanceCosts / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |
Shares of common stock allocated |  |  |  |  |  |  | 10,003,300 us-gaap_SharesIssued / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |
Accrued noncumulative dividends on preferred stock |  |  | 0.0341 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.14 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 6.95 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesBPreferredStockMember |  |  |  |  |  |  |  |  |  |
Series C Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  |  |  |  |  |  | 110,952,217 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |
Proceeds from issuance of convertible preferred stock |  |  |  |  |  |  | 44,900,000 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |
Related offering costs |  |  |  |  |  |  | 101,246 mgnx_StockIssuanceCosts / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |
Accrued noncumulative dividends on preferred stock |  |  | 0.0324 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.00 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 7.70 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesCPreferredStockMember |  |  |  |  |  |  |  |  |  |
Series D Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued noncumulative dividends on preferred stock |  |  | 0.0522 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesDPreferredStockMember |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.00 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesDPreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 12.20 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesDPreferredStockMember |  |  |  |  |  |  |  |  |  |
Series D Preferred Stock [Member] | Raven Biotechnologies Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares issued to acquire entity |  |  |  |  |  |  |  | 14,446,227 us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued / us-gaap_BusinessAcquisitionAxis = mgnx_RavenBiotechnologiesIncMember / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesDPreferredStockMember | 12,466,039 us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued / us-gaap_BusinessAcquisitionAxis = mgnx_RavenBiotechnologiesIncMember / us-gaap_StatementClassOfStockAxis = us-gaap_SeriesDPreferredStockMember |  |  |  |
Series D-2 Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares issued or sold |  |  |  |  |  |  |  |  |  | 6,916,110 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember | 38,337,678 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |
Proceeds from issuance of convertible preferred stock |  |  |  |  |  |  |  |  |  |  | 24,800,000 us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |
Related offering costs |  |  |  |  |  |  |  |  |  |  | $ 156,788 mgnx_StockIssuanceCosts / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |
Purchased shares |  |  |  |  |  |  |  |  |  | 518,708 us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember | 2,875,327 us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |
Exercise price |  |  | 0.65 us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1 / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Accrued noncumulative dividends on preferred stock |  |  | 0.0522 us-gaap_PreferredStockDividendRatePerDollarAmount / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Convertible preferred stock series |  |  | 1.00 us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Conversion price of preferred stock |  |  | 12.20 mgnx_ConvertiblePreferredStockConversionPrice / us-gaap_StatementClassOfStockAxis = mgnx_SeriesD2PreferredStockMember |  |  |  |  |  |  |  |  |  |
Undesignated Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Undesignated preferred stock, shares authorized |  |  | 5,000,000 us-gaap_PreferredStockSharesAuthorized / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |  |  |  |  |  |  |  |  |  |
Undesignated preferred stock, par value |  |  | 0.01 us-gaap_PreferredStockParOrStatedValuePerShare / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |  |  |  |  |  |  |  |  |  |
Undesignated preferred stock, shares issued |  |  | 0 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |  |  |  |  |  |  |  |  | 0 us-gaap_PreferredStockSharesIssued / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |
Undesignated preferred stock, shares outstanding |  |  | 0 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |  |  |  |  |  |  |  |  | 0 us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = mgnx_UndesignatedPreferredStockMember |
Series A-1 Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Shares of common stock allocated |  |  |  |  |  | 13,604,016 us-gaap_SharesIssued / us-gaap_StatementClassOfStockAxis = mgnx_SeriesAOnePreferredStockMember |  |  |  |  |  |  |

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