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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 | 1,800,000 us-gaap_StockIssuedDuringPeriodSharesNewIssues |  |  |
Stock issued during period value, new issues |  | $ 76,716,000 us-gaap_StockIssuedDuringPeriodValueNewIssues | $ 83,623,000 us-gaap_StockIssuedDuringPeriodValueNewIssues |
Janssen Biotech Inc. [Member] |  |  |  |
Subsequent Event [Line Items] |  |  |  |
Stock issued during the period for new issue |  | 1,923,077 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember |  |
Stock issued during period value, new issues |  | 75,000,000 us-gaap_StockIssuedDuringPeriodValueNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember |  |
Nonrefundable upfront payment receivable |  | 50,000,000 mgnx_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 |  |  |  | 1,923,077 us-gaap_StockIssuedDuringPeriodSharesNewIssues / us-gaap_CounterpartyNameAxis = mgnx_JohnsonAndJohnsonDevelopmentCorporationMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Stock issued during period value, new issues |  |  |  | 75,000,000 us-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 |  |  |  | $ 50,000,000 mgnx_NonRefundableUpfrontPaymentReceivable / us-gaap_CounterpartyNameAxis = mgnx_JanssenBiotechIncMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember

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- 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 -Article 5 + Details
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