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Related Party Balances and Transactions - Summary of Related Party Transactions (Detail) ¥ in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2020 CNY (¥)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 CNY (¥)
CMAB Biopharma Suzhou Inc [Member] | Project Development Fee [Member]
Related Party Transaction [Line Items]
Research and development expenses recognized	¥ 690	$ 102	¥ 0
Tasly Pharmaceutical Group Co Ltd [Member] | Receipt of CRO Services [Member]
Related Party Transaction [Line Items]
Research and development expenses recognized	0	0	5,590
Everest [Member]
Related Party Transaction [Line Items]
Receipt of research and development funding	¥ 0	$ 0	¥ 52,207

X

- Definition

Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

+ Details

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Balance Type:	debit
Period Type:	duration

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

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X

- Definition

Reflects the sum of all other revenue and income realized from sales and other transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party during the period.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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X

- Details

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X

- Details

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X

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=imab_TaslyPharmaceuticalGroupCoLtdMember
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X

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=imab_ReceiptOfContractResearchOrganizationsMember
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X

- Details

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