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Summary of significant accounting policies - Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Total net sales $ 59,055 $ 55,856 $ 124,975 $ 116,430
United States        
Disaggregation of Revenue [Line Items]        
Total net sales 53,038 49,906 115,145 106,329
International        
Disaggregation of Revenue [Line Items]        
Total net sales $ 6,017 $ 5,950 $ 9,830 $ 10,101