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Accounts receivable, net - Movement of allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Accounts receivable, net      
Balance at the beginning of the year ¥ 86,417 ¥ 46,900 ¥ 43,084
Provision for the year 55,839 39,517 8,014
Write-off     (4,198)
Balance at the end of the year ¥ 142,256 ¥ 86,417 ¥ 46,900