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Product Milestone Payments and Royalty Agreements - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 30, 2005	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
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

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 shares which could be issued as additional consideration under asset purchase agreement.

+ References

No definition available.

+ Details

Name:	mgnx_NumberOfSharesIssuedAsAdditionalConsiderationUnderAssetPurchaseAgreement
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