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Commitments and Contingencies - Additional Information (Detail) | 6 Months Ended
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Jun. 30, 2014
Operating Leased Assets [Line Items] |
Lessee leasing arrangements, operating leases, renewal term | 5 years
Manufacturing Facilities [Member] |
Operating Leased Assets [Line Items] |
Term of additional space lease | 4 years
Amended lease expiration date | Dec. 31, 2019
Origination date for additional space in manufacturing facility | Apr. 01, 2014
Lease [Member] | Office and Laboratory Space [Member] |
Operating Leased Assets [Line Items] |
Lease expiration date | Jun. 30, 2018
Lease [Member] | Manufacturing Facilities [Member] |
Operating Leased Assets [Line Items] |
Lease expiration date | Dec. 31, 2014
Sublease [Member] | Office and Laboratory Space [Member] |
Operating Leased Assets [Line Items] |
Lease expiration date | Dec. 31, 2018

X
- Definition Amended Lease Expiration Date + References No definition available. + Details
Name: | mgnx_AmendedLeaseExpirationDate
Namespace Prefix: | mgnx_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Lease Agreement Initiation Date + References No definition available. + Details
Name: | mgnx_LeaseAgreementInitiationDate
Namespace Prefix: | mgnx_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date which lease or group of leases is set to expire, in CCYY-MM-DD format. + References No definition available. + Details
Name: | us-gaap_LeaseExpirationDate1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Term of the lessee's leasing arrangement renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_LesseeLeasingArrangementsOperatingLeasesRenewalTerm
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Term of the lessee's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_OperatingLeasedAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration