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Product Milestone Payments and Royalty Agreements - Additional Information (Detail) (USD $) | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jun. 30, 2005 | Sep. 30, 2013 | Sep. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2011
Royalty Agreement [Line Items] |  |  |  |  |
Additional consideration of asset purchase agreement | $ 10,950,000 |  |  |  |
Common stock shares | 36,135 |  |  |  |
Payments related to the additional considerations |  | $ 0 | $ 0 | $ 0 | $ 0
Minimum [Member] |  |  |  |  |
Royalty Agreement [Line Items] |  |  |  |  |
Maximum royalty payments percentage | 1.75% |  |  |  |
Maximum [Member] |  |  |  |  |
Royalty Agreement [Line Items] |  |  |  |  |
Maximum royalty payments percentage | 4.00% |  |  |  |

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 Potential Additional Payment Upon Achievement Of Milestone + References No definition available. + Details
Name: | mgnx_PotentialAdditionalPaymentUponAchievementOfMilestone
Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Potential Number Of Shares Issued As Additional Consideration Under Asset Purchase Agreement + References No definition available. + Details
Name: | mgnx_PotentialNumberOfSharesIssuedAsAdditionalConsiderationUnderAssetPurchaseAgreement
Namespace Prefix: | mgnx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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