Portion of consideration allocated to equity.

No definition available.

Name:	mgnx_AmountAllocatedToEquity
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Data Type:	xbrli:monetaryItemType
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- Definition

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

+ References

No definition available.

+ Details

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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

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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

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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

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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

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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

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

- Definition

Proceeds From Research And Development Services

+ References

No definition available.

+ Details

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

- Definition

Total consideration received under agreement.

+ References

No definition available.

+ Details

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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

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

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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

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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

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- Details

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- Details

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- Details

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Collaboration and Other Agreements - Janssen Biotech, Inc. (Details) - Janssen Biotech, Inc. - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 31, 2016	Jan. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
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
Revenue recognized under agreement			$ 0	$ 0	$ 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		205,000,000.0
Potential regulatory milestone payments under agreement			220,000,000.0		220,000,000.0
Potential sales milestone payments under agreement			150,000,000.0		150,000,000.0
Janssen MGD015 Agreement
Collaboration and Other Agreements [Line Items]
Non-refundable upfront payment	$ 75,000,000
Revenue recognized under agreement			200,000	$ 0	300,000	$ 0
Janssen MGD015 Agreement \| Maximum
Collaboration and Other Agreements [Line Items]
Potential clinical milestone payments			100,000,000.0		100,000,000.0
Potential regulatory milestone payments under agreement			265,000,000.0		265,000,000.0
Potential sales milestone payments under agreement			$ 300,000,000.0		$ 300,000,000.0