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Prepayments and other assets - Allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Prepayment and other assets      
Balance at the beginning of the year ¥ 1,113 ¥ 1,411 ¥ 1,324
Provision (Reversal) for the year 3,142 (493) 2,701
Write-off     (2,614)
Collection of previously written-off debtors   195  
Balance at the end of the year ¥ 4,255 ¥ 1,113 ¥ 1,411