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Employee Benefit Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013
Retirement Benefits [Abstract]
Eligible age to participate in plan				21 years
Percentage of employee contribution to salary				100.00%
Percentage of employee vested contribution	100.00%
Defined contribution plan, employer discretionary contribution amount	$ 1.4	$ 1.3	$ 1.1

X

- Definition

Percentage vested of employee's contributions to defined contribution plan

+ References

No definition available.

+ Details

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

X

- Definition

Refers to minimum eligible age of employee to participate in benefit plan.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

Name:	us-gaap_CompensationAndRetirementDisclosureAbstract
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

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