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Summary of significant accounting policies - Additional Information (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
segment
Jun. 30, 2017
USD ($)
Jan. 01, 2018
USD ($)
Dec. 31, 2017
USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]            
Number of reportable segments | segment     1      
Number of operating segments | segment     1      
Retained earnings $ (525,088) $ (554,317) $ (525,088) $ (554,317)   $ (526,972)
Net sales 59,055 55,856 124,975 116,430    
Accounts receivable 27,639 $ 26,143 27,639 $ 26,143   $ 44,634
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606            
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]            
Retained earnings         $ (100)  
Net sales 200   700      
Accounts receivable $ 700   $ 700      
Minimum            
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]            
Payment terms     30 days      
Maximum            
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]            
Payment terms     60 days