Common Stock, Maximum Amount Available For Issuance

No definition available.

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Balance Type:	debit
Period Type:	duration

- Definition

Stock issued during period, shares to underwriters for exercise of over allotment option.

+ References

No definition available.

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
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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- Definition

Face amount or stated value per share of common stock.

+ References

+ Details

Name:	us-gaap_CommonStockParOrStatedValuePerShare
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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of common shares permitted to be issued by an entity's charter and bylaws.

+ References

+ Details

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Period Type:	instant

- Definition

Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer.

+ References

+ Details

Name:	us-gaap_PreferredStockParOrStatedValuePerShare
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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws.

+ References

+ Details

Name:	us-gaap_PreferredStockSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt.

+ References

+ Details

Name:	us-gaap_PreferredStockSharesIssued
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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Aggregate share number for all nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) held by stockholders. Does not include preferred shares that have been repurchased.

+ References

+ Details

Name:	us-gaap_PreferredStockSharesOutstanding
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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash inflow from the additional capital contribution to the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash inflow from the issuance of common stock, preferred stock, treasury stock, stock options, and other types of equity.

+ References

+ Details

Name:	us-gaap_ProceedsFromIssuanceOrSaleOfEquity
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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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
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Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

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

- Definition

Per share or per unit amount of equity securities issued.

+ References

No definition available.

+ Details

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

- Definition

Number of new stock issued during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
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- Details

Name:	us-gaap_StatementClassOfStockAxis=mgnx_UndesignatedPreferredStockMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=mgnx_RegisteredDirectOfferingMember
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- Details

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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended			12 Months Ended
	May 03, 2017	Apr. 26, 2017	Jul. 31, 2015	Jan. 31, 2015	Feb. 28, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized (in shares)						125,000,000	125,000,000
Common stock, par value (in dollars per share)						$ 0.01	$ 0.01
Number of shares issued or sold (in shares)			3,525,000		1,800,000
Price per share (in dollars per share)			$ 37.00		$ 36.50
Additional shares sold under over-allotments option to underwriter (in shares)			528,750		450,000
Proceeds from sale of shares			$ 141,000		$ 76,700
Proceeds from issuance of common stock, net of offering costs						$ 34,244	$ 0	$ 203,467
Johnson & Johnson Innovation JJDC, Inc,
Class of Stock [Line Items]
Sale of common stock (in shares)				1,923,077
Sale of common stock (in dollars per share)				$ 39
Proceeds of stock sale				$ 75,000
Undesignated Preferred Stock
Class of Stock [Line Items]
Undesignated preferred stock, shares authorized (in shares)						5,000,000
Undesignated preferred stock, par value (in dollars per share)						$ 0.01
Undesignated preferred stock, shares issued (in shares)						0	0
Undesignated preferred stock, shares outstanding (in shares)						0	0
Registered Direct Offering
Class of Stock [Line Items]
Number of shares issued or sold (in shares)		1,100,000
Common stock purchase price (in dollars per share)		$ 21.50
Proceeds from issuance of common stock, net of offering costs		$ 23,700
At The Market Offering
Class of Stock [Line Items]
Number of shares issued or sold (in shares)						599,284
Proceeds of stock sale						$ 10,800
Common stock maximum amount available for issuance	$ 75,000