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Income taxes - Other (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Taxes      
Increase (decrease) in Valuation allowance $ 9,400 $ (3,900)  
Unrecognized tax benefit 0    
Accrued interest or penalties 0 $ 0 $ 0
Federal      
Income Taxes      
Net operating loss (NOL) carryforwards 136,800    
Tax credit carryforwards 3,800    
State      
Income Taxes      
Net operating loss (NOL) carryforwards 108,800    
Tax credit carryforwards 1,200    
Foreign      
Income Taxes      
Net operating loss (NOL) carryforwards 75    
Aushon acquisition      
Income Taxes      
Net operating loss (NOL) carryforwards $ 900