Portion of consideration allocated to equity.

No definition available.

Name:	mgnx_AmountAllocatedToEquity
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Portion of consideration allocated to license and pre-clinical research and development.

+ References

No definition available.

+ Details

Name:	mgnx_AmountAllocatedToLicenseAndRD
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Initial fee received from collaboration or license agreement.

+ References

No definition available.

+ Details

Name:	mgnx_NonRefundableUpfrontFees
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Total potential clinical milestone payments company could earn under agreement.

+ References

No definition available.

+ Details

Name:	mgnx_PotentialClinicalMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total potential regulatory milestones company could earn under agreement

+ References

No definition available.

+ Details

Name:	mgnx_PotentialRegulatoryMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Total potential sales milestones company could earn under agreement

+ References

No definition available.

+ Details

Name:	mgnx_PotentialSalesMilestonePaymentsUnderAgreement
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount by which purchase price exceeds fair value for common stock purchased by collaborator.

+ References

No definition available.

+ Details

Name:	mgnx_PremiumReceivedOnStockPurchase
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Total consideration received under agreement.

+ References

No definition available.

+ Details

Name:	mgnx_TotalConsideration
Namespace Prefix:	mgnx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Revenue earned during the period relating to consideration received from another party for the right to use, but not own, certain of the entity's intangible assets. Licensing arrangements include, but are not limited to, rights to use a patent, copyright, technology, manufacturing process, software or trademark. Licensing fees are generally, but not always, fixed as to amount and not dependent upon the revenue generated by the licensing party. An entity may receive licensing fees for licenses that also generate royalty payments to the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1(e))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_LicensesRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockConsiderationReceivedOnTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The number of shares issued or sold by the subsidiary or equity method investee per stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockNumberOfSharesIssuedInTransaction
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Per share amount received by subsidiary or equity investee for each share of common stock issued or sold in the stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockPricePerShare
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_CounterpartyNameAxis=mgnx_JanssenBiotechIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis=mgnx_JanssenMGD011AgreementMember
Namespace Prefix:
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Collaboration and Other Agreements - Janssen Biotech, Inc. (Details) - Janssen Biotech, Inc. - USD ($)	1 Months Ended		3 Months Ended
	Jul. 31, 2016	Jan. 31, 2015	Mar. 31, 2017	Mar. 31, 2016
Janssen MGD015 Agreement
Collaboration and Other Agreements [Line Items]
Non-refundable upfront payment	$ 75,000,000
Recognized revenue under agreement			$ 100,000.0	$ 0
Janssen MGD015 Agreement \| Maximum
Collaboration and Other Agreements [Line Items]
Potential clinical milestone payments			100,000,000.0
Potential regulatory milestone payments under agreement			265,000,000.0
Potential sales milestone payments under agreement			300,000,000.0
Janssen MGD011 Agreement
Collaboration and Other Agreements [Line Items]
Sale of common stock (in shares)		1,923,077
Proceeds of stock sale		$ 75,000,000
Non-refundable upfront payment		$ 50,000,000
Recognized revenue under agreement			0	$ 0
Sale of common stock (in dollars per share)		$ 39.00
Premium received on stock purchase		$ 12,300,000
Total consideration		125,000,000
Amount allocated to equity		62,700,000
Amount allocated to license and R&D		$ 62,300,000
Janssen MGD011 Agreement \| Maximum
Collaboration and Other Agreements [Line Items]
Potential clinical milestone payments			205,000,000.0
Potential regulatory milestone payments under agreement			220,000,000.0
Potential sales milestone payments under agreement			$ 150,000,000.0