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Trade and other receivables, net - Charged into restricted bank accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Trade and other receivables, net      
Beginning balance $ 2,510 $ 2,782 $ 2,372
Increase in restricted bank accounts   166 410
Decrease in restricted bank accounts (2,134) (438) 0
Final balance $ 376 $ 2,510 $ 2,782