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Product Milestone Payments and Royalty Agreements - Additional Information (Detail) (USD $) | 12 Months Ended | 36 Months Ended | 1 Months Ended
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2014 | Jun. 30, 2005
Royalty Agreement [Line Items] |  |  |
Additional consideration of asset purchase agreement |  |  | 10,950,000 mgnx_AdditionalPaymentUponAchievementOfMilestone
Common stock shares |  |  | 36,135 mgnx_NumberOfPotentialSharesToBeIssuedAsAdditionalConsiderationUnderAssetPurchaseAgreement
Payments related to the additional considerations |  | 0 mgnx_AmountOfAdditionalConsiderationPaidUnderAgreement |
Revenue from product sales | $ 0 us-gaap_SalesRevenueGoodsNet |  |
Minimum [Member] |  |  |
Royalty Agreement [Line Items] |  |  |
Maximum royalty payments percentage |  |  | 1.75%mgnx_RoyaltyPaymentAsPercentageOfNetSalesUnderLicensingAgreement / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member] |  |  |
Royalty Agreement [Line Items] |  |  |
Maximum royalty payments percentage |  |  | 4.00%mgnx_RoyaltyPaymentAsPercentageOfNetSalesUnderLicensingAgreement / us-gaap_RangeAxis = us-gaap_MaximumMember

X
- Definition Additional payment upon achievement of milestone. + References No definition available. + Details
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Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of additional consideration paid under agreement. + References No definition available. + Details
Name: | mgnx_AmountOfAdditionalConsiderationPaidUnderAgreement
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of potential shares to be issued as additional consideration under asset purchase agreement. + References No definition available. + Details
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Namespace Prefix: | mgnx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Details
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Namespace Prefix: | mgnx_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Royalty payment as percentage of net sales under licensing agreement. + References No definition available. + Details
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- Definition Aggregate revenue during the period from the sale of goods in the normal course of business, after deducting returns, allowances and discounts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1(a)) -URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Article 5 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |