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Lease Exit Liability - Additional Information (Detail) (USD $) | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2014 | Jun. 30, 2013 | Jun. 30, 2014 | Jun. 30, 2013 | Jul. 16, 2008
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Operating Lease |  |  | 2018 |  |
Onetime purchase payment under purchase agreement |  |  |  |  | $ 5,000,000
Incremental in-process research and development expense | 0 | 0 | 0 | 0 |
Minimum [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Onetime payment paid under license of product |  |  |  |  | 8,000,000
Maximum [Member] |  |  |  |  |
Restructuring Cost and Reserve [Line Items] |  |  |  |  |
Onetime payment paid under license of product |  |  |  |  | $ 12,000,000

X
- Definition Contingent consideration recorded as incremental in process research and development expense. + References No definition available. + Details
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X
- Definition Lease expiration year. + References No definition available. + Details
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Balance Type: | na
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X
- Definition One-time milestone payment to be paid under agreement. + References No definition available. + Details
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Namespace Prefix: | mgnx_
Data Type: | xbrli:monetaryItemType
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- Definition One-time milestone payment to be paid under agreement upon specified level of sales of products. + References No definition available. + Details
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- Details
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