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Summary of significant accounting policies - Concentration of risk (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
USD ($)
item
Jun. 30, 2019
item
Jun. 30, 2020
USD ($)
item
Jun. 30, 2019
item
Dec. 31, 2019
USD ($)
Concentration Risk [Line Items]          
Trade accounts receivable, net $ 53,275   $ 53,275   $ 46,513
Revenue from Contract with Customer Benchmark [Member] | Credit Concentration Risk [Member]          
Concentration Risk [Line Items]          
Number of significant distributors | item 2 1 2 1  
Accounts Receivable [Member] | Credit Concentration Risk [Member]          
Concentration Risk [Line Items]          
Trade accounts receivable, net $ 19,300   $ 19,300   $ 6,000