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Employee Benefit Plan - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2002 Age	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Eligibility of employee participation plan	21
Percentage of employee contribution to salary	100.00%
Percentage of employee vested contribution		100.00%
Company's contribution to plan total		$ 194,498	$ 225,195	$ 217,097

X

- Definition

Defined contribution plan, employees contribution, annual vesting percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	mgnx_
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Balance Type:	na
Period Type:	duration

X

- Definition

Employee eligible age to participate in benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	mgnx_
Data Type:	xbrli:integerItemType
Balance Type:	na
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X

- Details

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Namespace Prefix:	mgnx_
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

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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
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