Organization and Nature of Operations - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	9 Months Ended	1 Months Ended
Sep. 30, 2013 Segment	Oct. 31, 2013 Subsequent Event [Member] IPO [Member]	Oct. 16, 2013 Subsequent Event [Member] IPO [Member]
Organization And Nature Of Business [Line Items]
MacroGenics incorporated date	Aug. 14, 2000
Number of operating segments	1
Initial public offering number of share sold		5,750,000
Initial public offering price per share			$ 16
Proceeds from initial public offering before offering expenses		$ 85.6

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.

No definition available.

Name:	us-gaap_NumberOfOperatingSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration