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Stock- based Compensation - Additional Information (Detail) (USD $) | 12 Months Ended | 60 Months Ended |  |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2010 | Feb. 28, 2003 | Oct. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number | 3,572,116 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber | 3,200,958 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber |  |  |  |
Weighted average exercise price | $ 11.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice | $ 4.90 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice |  |  |  |
Expected life of option term | 6 years 3 months | 7 years | 7 years |  |  |
Common stock, shares issued | 568,906 us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised | 1,268,049 us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised | 863,176 us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised |  |  |
Cash proceeds from exercise of stock options | $ 744,000 us-gaap_ProceedsFromStockOptionsExercised | $ 1,100,000 us-gaap_ProceedsFromStockOptionsExercised | $ 47,000 us-gaap_ProceedsFromStockOptionsExercised |  |  |
Weighted-average grant-date fair value of options granted | $ 17.41 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 6.91 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue | $ 0.94 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue |  |  |
Share-based compensation arrangement by share-based payment award, options, exercises in period, intrinsic value | 14,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | 5,400,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | 271,929 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue |  |  |
Fair value of shares vested | 3,000,000 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 | 487,603 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 | 879,024 us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1 |  |  |
Unrecognized compensation cost related to non-vested stock-based compensation arrangements | $ 19,600,000 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized |  |  |  |  |
Unrecognized compensation expense recognition period | 4 years |  |  |  |  |
Maximum [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options granted, maximum term | 10 years |  |  |  |  |
Stock Option Plan 2000 [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized | 130,725 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member |  |  |  |  |
Increased number of shares authorized | 150,297 mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number | 19,705 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member |  |  |  |  |
Weighted average exercise price | $ 0.81 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member |  |  |  |  |
Equity Incentive Plan 2003 [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized |  |  |  |  | 2,051,644 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member |
Increased number of shares authorized |  |  | 4,336,730 mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member | 3,790,760 mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number | 2,008,474 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member |  |  |  |  |
Weighted average exercise price | $ 1.64 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member |  |  |  |  |
Increase in aggregate number of shares authorized |  |  | 545,970 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member | 1,739,116 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member |  |
Stock Incentive Plan 2013 [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Share-based compensation, number of shares authorized |  |  |  |  |  | 1,960,168 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member
Increased number of shares authorized | 2,967,272 mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member |  |  |  |  |
Share-based compensation arrangement by share-based payment award, options, outstanding, number | 1,543,937 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member |  |  |  |  |
Weighted average exercise price | $ 24.22 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member |  |  |  |  |
Stock reserved for future issuance | 1,960,168 mgnx_CommonStockCapitalSharesMinimumIncreaseInSharesReservedForFutureIssuance / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member |  |  |  |  |
Percentage of common stock share outstanding | 4.00%mgnx_PercentageOfSharesOfCompanysCommonStockOutstandingForIncreaseInSharesReservedForIssuanceUnderStockPlan / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member |  |  |  |  |

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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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- Definition Number of additional shares authorized for issuance under an established share-based compensation plan. + References No definition available. + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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