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Related Party Balances and Transactions - Summary of Related Party Balances (Detail) ¥ in Thousands, $ in Thousands | Sep. 30, 2020 CNY (¥) | Sep. 30, 2020 USD ($) | Dec. 31, 2019 CNY (¥)
--- | --- | --- | ---
Related Party Transaction [Line Items] |  |  |
Ordinary shares to be issued to Everest |  |  | ¥ 258,119
Loans to an affiliate | ¥ 52,000 | $ 7,659 |
I-Mab Hangzhou [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Loans to an affiliate | 52,000 | 7,659 | 0
Everest [Member] |  |  |
Related Party Transaction [Line Items] |  |  |
Ordinary shares to be issued to Everest | ¥ 0 | $ 0 | ¥ 258,119

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