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Stock- based Compensation - Additional Information (Detail) (USD $)	12 Months Ended			60 Months Ended
	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	3,572,116us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	3,200,958us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Weighted average exercise price	$ 11.40us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	$ 4.90us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Expected life of option term	6 years 3 months	7 years	7 years
Common stock, shares issued	568,906us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	1,268,049us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised	863,176us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Cash proceeds from exercise of stock options	$ 744,000us-gaap_ProceedsFromStockOptionsExercised	$ 1,100,000us-gaap_ProceedsFromStockOptionsExercised	$ 47,000us-gaap_ProceedsFromStockOptionsExercised
Weighted-average grant-date fair value of options granted	$ 17.41us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	$ 6.91us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue	$ 0.94us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Share-based compensation arrangement by share-based payment award, options, exercises in period, intrinsic value	14,500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	5,400,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue	271,929us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Fair value of shares vested	3,000,000us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1	487,603us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1	879,024us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1
Unrecognized compensation cost related to non-vested stock-based compensation arrangements	$ 19,600,000us-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	130,725us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member
Increased number of shares authorized	150,297mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member
Share-based compensation arrangement by share-based payment award, options, outstanding, number	19,705us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_StockOptionPlan2000Member
Weighted average exercise price	$ 0.81us-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					2,051,644us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member
Increased number of shares authorized			4,336,730mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member	3,790,760mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member
Share-based compensation arrangement by share-based payment award, options, outstanding, number	2,008,474us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member
Weighted average exercise price	$ 1.64us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member
Increase in aggregate number of shares authorized			545,970us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAdditionalSharesAuthorized / us-gaap_PlanNameAxis = mgnx_EquityIncentivePlan2003Member	1,739,116us-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						1,960,168us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member
Increased number of shares authorized	2,967,272mgnx_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorizedAfterIncrease / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member
Share-based compensation arrangement by share-based payment award, options, outstanding, number	1,543,937us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member
Weighted average exercise price	$ 24.22us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_PlanNameAxis = mgnx_StockIncentivePlan2013Member
Stock reserved for future issuance	1,960,168mgnx_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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Unrecognized cost of unvested share-based compensation awards.

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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.

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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.

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Number of additional shares authorized for issuance under an established share-based compensation plan.

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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.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

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Number of share options (or share units) exercised during the current period.

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