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Commitments and Contingencies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2014
Operating Leases [Line Items]
Lessee leasing arrangements, operating leases, renewal term	5 years
Lease [Member] | Office and Laboratory Space [Member]
Operating Leases [Line Items]
Lease expiration date	Mar. 31, 2018
Lease [Member] | Manufacturing Facilities [Member]
Operating Leases [Line Items]
Lease expiration date	Dec. 31, 2014
Sublease [Member] | Office and Laboratory Space [Member]
Operating Leases [Line Items]
Lease expiration date	Dec. 31, 2018

X

- Details

Name:	mgnx_OperatingLeasesLineItems
Namespace Prefix:	mgnx_
Data Type:	xbrli:stringItemType
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