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Summary of significant accounting policies - Disaggregation of Revenue (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Disaggregation of Revenue [Line Items]    
Total net sales $ 65,920 $ 60,574
United States    
Disaggregation of Revenue [Line Items]    
Total net sales 62,107 56,423
International    
Disaggregation of Revenue [Line Items]    
Total net sales $ 3,813 $ 4,151