FINANCIAL RISK MANAGEMENT - Sensitivity analysis (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disclosure Of financial risk management [Line Items]
Loss before taxation	¥ 193,062	¥ 9,445	¥ 418,465
Currency risk [member]
Disclosure Of financial risk management [Line Items]
Loss before taxation	¥ 421	¥ 414	¥ 353

The profit (loss) before tax expense or income. [Refer: Profit (loss)]

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