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Stock-based compensation - Summary of stock-based compensation expense (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Share-based compensation $ 3,338 $ 3,347
Cost of goods sold    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Share-based compensation 601 650
Research and development    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Share-based compensation 881 773
Sales and marketing    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Share-based compensation 678 938
General and administrative    
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]    
Share-based compensation $ 1,178 $ 986