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Trade and other receivables, net - Charged into restricted bank accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Trade and other receivables, net      
Beginning balance $ 2,782 $ 2,372 $ 2,087
Increase in restricted bank accounts 166 410 285
Decrease in restricted bank accounts (438)   0
Final balance $ 2,510 $ 2,782 $ 2,372