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Subsequent Event - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended		1 Months Ended
	Feb. 28, 2014	Dec. 31, 2014	Dec. 31, 2013	Jan. 31, 2015
Subsequent Event [Line Items]
Stock issued during the period for new issue	1,800,000us-gaap_StockIssuedDuringPeriodSharesNewIssues
Stock issued during period value, new issues		$ 76,716,000us-gaap_StockIssuedDuringPeriodValueNewIssues	$ 83,623,000us-gaap_StockIssuedDuringPeriodValueNewIssues
Janssen Biotech Inc. [Member]
Subsequent Event [Line Items]
Stock issued during the period for new issue		1,923,077us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember
Stock issued during period value, new issues		75,000,000us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember
Nonrefundable upfront payment receivable		50,000,000mgnx_NonRefundableUpfrontPaymentReceivable / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember
Subsequent Event [Member] | Johnson & Johnson Innovation JJDC, Inc, [Member]
Subsequent Event [Line Items]
Stock issued during the period for new issue				1,923,077us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JohnsonAndJohnsonDevelopmentCorporationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Stock issued during period value, new issues				75,000,000us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JohnsonAndJohnsonDevelopmentCorporationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Subsequent Event [Member] | Janssen Biotech Inc. [Member]
Subsequent Event [Line Items]
Nonrefundable upfront payment receivable				$ 50,000,000mgnx_NonRefundableUpfrontPaymentReceivable / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

X

- Definition

Initial fee receivable from collaboration or license agreement.

+ References

No definition available.

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X

- Definition

Number of new stock issued during the period.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 29, 30

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- Definition

Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Article 3



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.3-04)

-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

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