Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim consolidated financial statements of MacroGenics, Inc. (the Company) have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information. The financial statements include all adjustments (consisting only of normal recurring adjustments) that the management of the Company believes are necessary for a fair presentation of the periods presented. These interim financial results are not necessarily indicative of results expected for the full fiscal year or for any subsequent interim period. The accompanying unaudited interim consolidated financial statements include the accounts of MacroGenics, Inc. and its wholly owned subsidiaries, MacroGenics UK Limited and MacroGenics Limited. All intercompany accounts and transactions have been eliminated in consolidation. These consolidated financial statements and related notes should be read in conjunction with the financial statements and notes thereto included in the Company's 2019 Annual Report on Form 10-K filed with the Securities and Exchange Commission (SEC) on February 25, 2020.
Recently Adopted Accounting Standards	In June 2016, the Financial Accounting Standards Board (FASB) issued ASU 2016-13, which modifies the measurement of expected credit losses on certain financial instruments. In addition, for available-for-sale debt securities, the standard eliminates the concept of other-than-temporary impairment and requires the recognition of an allowance for credit losses rather than reductions in the amortized cost of the securities. The Company adopted ASU 2016-13 and all related ASU amendments on January 1, 2020, using a modified retrospective transition method, which requires a cumulative-effect adjustment, if any, to the opening balance of retained earnings to be recognized on the date of adoption with prior periods not restated. The Company evaluated its available-for-sale debt securities at January 1, 2020 and determined that no cumulative-effect adjustment was required. Adoption of the new standard did not have a material impact on the Company's consolidated financial statements. Under the new guidance, at each reporting date, entities must evaluate their individual available-for-sale debt securities that are in an unrealized loss position and determine whether the decline in fair value below the amortized cost basis results from a credit loss or other factors. The Company evaluates various quantitative factors including, but not limited to, the nature of the investments, changes in credit ratings, interest rate fluctuations, industry analyst reports, and the severity of the impairment. The amount of the decline related to credit losses is recorded as a credit loss expense in earnings with a corresponding allowance for credit losses and the amount of the decline not related to credit losses is recorded through other comprehensive income. See Note 4, Marketable Securities, for additional information. In August 2018, the FASB issued ASU No. 2018-15, Intangibles - Goodwill and Other - Internal-Use Software (ASU 2018-15). This new standard requires a customer in a cloud computing arrangement that is a service contract to follow the internal-use software guidance in ASC 350-40, Accounting for Internal-Use Software, to determine which implementation costs to capitalize as assets and amortize over the term of the hosting arrangement or expense as incurred. The Company adopted ASU 2018-15 on January 1, 2020 on a prospective basis. Adoption of the new standard did not have a material impact on the Company's consolidated financial statements. In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808)—Clarifying the interaction between Topic 808 and Topic 606 (ASU 2018-18). The amendments provide guidance on whether certain transactions between collaborative arrangement participants should be accounted for as revenue under ASC 606. It also specifically (i) addresses when the participant should be considered a customer in the context of a unit of account, (ii) adds unit-of-account guidance in ASC 808 to align with guidance in ASC 606, and (iii) precludes presenting revenue from a collaborative arrangement together with revenue recognized under ASC 606 if the collaborative arrangement participant is not a customer. The Company adopted ASU 2018-18 on January 1, 2020 on a retrospective basis. Adoption of the new standard did not have a material impact on the Company's consolidated financial statements. The Company has evaluated all other ASUs issued through the date the consolidated financials were issued and believes that the adoption of these ASUs will not have a material impact on the Company's consolidated financial statements