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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Impairment of long-lived assets (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES      
Impairment losses for long-lived assets ¥ 0 ¥ 0 ¥ 0