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Summary of Significant Accounting Policies - Estimated Useful Lives (Detail)	9 Months Ended
	Sep. 30, 2013
Computer Equipment [Member]
Depreciation Amortization Impairment [Line Items]
Estimated Useful Lives	3 years
Software [Member]
Depreciation Amortization Impairment [Line Items]
Estimated Useful Lives	3 years
Furniture [Member]
Depreciation Amortization Impairment [Line Items]
Estimated Useful Lives	10 years
Laboratory and Office Equipment [Member]
Depreciation Amortization Impairment [Line Items]
Estimated Useful Lives	5 years
Leasehold Improvements [Member]
Depreciation Amortization Impairment [Line Items]
Leasehold improvements	Shorter of lease term or useful life

X

- Details

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

X

- Definition

Describes the periods of time over which an entity anticipates to receive utility from its property, plant and equipment (that is, the periods of time over which an entity allocates the initial cost of its property, plant and equipment).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367646&loc=d3e18780-107790



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-URI http://asc.fasb.org/subtopic&trid=2155824

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

X

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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