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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Segment Assets	Dec. 31, 2013 Assets
Revenue, Major Customer [Line Items]
Effective date of merger	Dec. 22, 2014
Number of operating segments	1us-gaap_NumberOfOperatingSegments
Original maturity of financial instruments	3 months	3 months
Allowance recorded	$ 0us-gaap_AllowanceForDoubtfulAccountsReceivable	$ 0us-gaap_AllowanceForDoubtfulAccountsReceivable
Amounts classified as restricted cash	$ 300,000us-gaap_RestrictedCashAndCashEquivalents	$ 405,000us-gaap_RestrictedCashAndCashEquivalents
Number of impaired assets	0mgnx_NumberOfImpairedAssets	0mgnx_NumberOfImpairedAssets
Number of assets held-for-sale	0mgnx_NumberOfAssetsHeldForSale	0mgnx_NumberOfAssetsHeldForSale
Minimum [Member] | Sales Revenue, Net [Member]
Revenue, Major Customer [Line Items]
Percentage of all significant revenue earned	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_SalesRevenueNetMember / us-gaap_RangeAxis = us-gaap_MinimumMember
Minimum [Member] | Accounts Receivable [Member]
Revenue, Major Customer [Line Items]
Percentage of all significant revenue earned	10.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = us-gaap_AccountsReceivableMember / us-gaap_RangeAxis = us-gaap_MinimumMember

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 4

-Article 5



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Topic 310

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-Name Accounting Standards Codification

-Topic 825

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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