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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset retirement costs (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Activity of the asset retirement obligation liability      
Asset retirement obligations at beginning of the year ¥ 52,441 ¥ 35,879 ¥ 17,648
Additions 20,384 13,572 16,391
Accretion expense 4,084 2,990 1,840
Asset retirement obligations at end of the year ¥ 76,909 ¥ 52,441 ¥ 35,879