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Significant accounting policies (cash and cash equivalents and trade accounts receivable) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Cash and cash equivalents    
Maximum maturity of cash equivalents 3 months  
Book overdrafts $ 5,665 $ 7,764
Trade accounts receivable    
Period for write-off of trade accounts receivable 1 year