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Accounts receivable, net - Schedule of Allowance for Doubtful Accounts (Details) - USD ($)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Accounts receivable, net      
Balance at beginning of the year $ 9,329,000 $ 7,604,000 $ 7,709,000
Additions 72,000 1,137,000 19,000
Reversals (481,000)    
Write-off (7,375,000)    
Exchange difference 219,000 588,000 (124,000)
Balance at end of the year $ 1,764,000 $ 9,329,000 $ 7,604,000