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Employee Benefit Plan - Additional Information (Detail) (USD $)	0 Months Ended	12 Months Ended
	Sep. 25, 2002	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Eligibility of employee participation Plan	21 years
Percentage of employee contribution to salary	100.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Percentage of employee vested contribution		100.00%mgnx_DefinedContributionPlanEmployeesContributionAnnualVestingPercentage
Defined contribution plan, employer discretionary contribution amount		$ 318,390us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount	$ 252,930us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount	$ 225,195us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount

X

- Definition

Defined contribution plan, employees contribution, annual vesting percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	mgnx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Employees eligible age to participate in benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	mgnx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration