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Principal Activities and Organization - Schedule of Principal Subsidiaries (Detail)	9 Months Ended
	Sep. 30, 2020
I-Mab Biopharma Hong Kong Limited [Member]
Place of incorporation	Hong Kong
Date of incorporation	Jul. 08, 2016
Percentage of direct or indirect ownership by the company	100.00%
Principal activities	Investment holding
I-Mab Shanghai [Member]
Place of incorporation	PRC
Date of incorporation	Aug. 24, 2016
Percentage of direct or indirect ownership by the company	100.00%
Principal activities	Research and development of innovative medicines
I-MabTianjin [Member]
Place of incorporation	PRC
Date of incorporation	Jul. 15, 2017
Percentage of direct or indirect ownership by the company	100.00%
Principal activities	Research and development of innovative medicines
I-MabBiopharma US Ltd. [Member]
Place of incorporation	U.S.
Date of incorporation	Feb. 28, 2018
Percentage of direct or indirect ownership by the company	100.00%
Principal activities	Research and development of innovative medicines

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A description of the principal activities of an investee accounted for under the equity method.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

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