XML 12 R26.htm IDEA: XBRL DOCUMENT
v3.19.3

Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 9 Months Ended
--- | --- | ---
Feb. 28, 2019 | Apr. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018
Class of Stock [Line Items] |  |  |  |
Proceeds from issuance of common stock, net of offering costs |  |  | $ 118,657 | $ 103,259
Firm Commitment Public Underwritten Offer |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued or sold (in shares) |  | 4,500,000 |  |
Common stock purchase price (in dollars per share) |  | $ 21.25 |  |
Proceeds from issuance of common stock, net of offering costs |  | $ 103,000 |  |
Over-allotment option |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued or sold (in shares) | 825,000 | 675,000 |  |
Common stock purchase price (in dollars per share) | $ 20.00 | $ 21.25 |  |
Follow-On Equity Offering |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares issued or sold (in shares) | 5,500,000 |  |  |
Common stock purchase price (in dollars per share) | $ 20.00 |  |  |
Proceeds from issuance of common stock, net of offering costs | $ 118,700 |  |  |

X
- 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_ClassOfStockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The cash inflow from the additional capital contribution to the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
Name: | us-gaap_ProceedsFromIssuanceOfCommonStock
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- Definition Number of new stock issued during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -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 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -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 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesNewIssues
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=mgnx_FirmCommitmentPublicUnderwrittenOfferMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=us-gaap_OverAllotmentOptionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SubsidiarySaleOfStockAxis=mgnx_FollowOnEquityOfferingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |