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Accounts receivable, net - Movements of the allowance for doubtful accounts (Details) - CNY (¥)
¥ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at the beginning of the year ¥ 14,175 ¥ 491 ¥ 418
Additions charged to bad debt expense 13,654 13,684 491
Write-off of bad debt allowance (4,531) 0 (418)
Balance at the end of the year ¥ 23,298 ¥ 14,175 ¥ 491