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Product Milestone Payments and Royalty Agreements - Additional Information (Detail) (USD $) | 36 Months Ended |  | 1 Months Ended
--- | --- | --- | ---
Dec. 31, 2013 | Jun. 30, 2005 | Jun. 30, 2005 Minimum [Member] | Jun. 30, 2005 Maximum [Member]
Royalty Agreement [Line Items] |  |  |  |
Additional consideration of asset purchase agreement |  | $ 10,950,000 |  |
Common stock shares |  | 36,135 |  |
Maximum royalty payments percentage |  |  | 1.75% | 4.00%
Payments related to the additional considerations | $ 0 |  |  |

X
- Definition Additional payment upon achievement of milestone. + References No definition available. + Details
Name: | mgnx_AdditionalPaymentUponAchievementOfMilestone
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
Name: | mgnx_NumberOfPotentialSharesToBeIssuedAsAdditionalConsiderationUnderAssetPurchaseAgreement
Namespace Prefix: | mgnx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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

X
- Definition Royalty payment as percentage of net sales under licensing agreement. + References No definition available. + Details
Name: | mgnx_RoyaltyPaymentAsPercentageOfNetSalesUnderLicensingAgreement
Namespace Prefix: | mgnx_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration