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Fair value of financial instruments - Summary of Fair Value of Financial Liabilities (Details) - Fair Value Measurements, Recurring Basis - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Financial liabilities:    
Long-term debt, including current portion $ 156,792 $ 165,393
Total financial liabilities 156,792 165,393
Current portion of long-term debt 8,646 8,650
Level 1    
Financial liabilities:    
Long-term debt, including current portion 0 0
Total financial liabilities 0 0
Level 2    
Financial liabilities:    
Long-term debt, including current portion 156,792 165,393
Total financial liabilities 156,792 165,393
Level 3    
Financial liabilities:    
Long-term debt, including current portion 0 0
Total financial liabilities $ 0 $ 0