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Fair value measurements (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Fair value measurements    
Increase (decrease) in fair value of non-financial assets $ 0  
Increase (decrease) in fair value of non-financial liabilities 0  
Cash flow hedges | Other long-term liabilities    
Fair value measurements    
Interest rate swap 769  
Cash flow hedges | Other long-term liabilities | Level 2    
Fair value measurements    
Interest rate swap $ 769  
Cash flow hedges | Other assets    
Fair value measurements    
Interest rate swap   $ 1,619
Cash flow hedges | Other assets | Level 2    
Fair value measurements    
Interest rate swap   $ 1,619