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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, 2018
Dec. 31, 2017
Financial liabilities:    
Long-term debt, including current portion $ 150,719 $ 156,792
Total financial liabilities 150,719 156,792
Current portion of long-term debt 9,861 8,646
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 150,719 156,792
Total financial liabilities 150,719 156,792
Level 3    
Financial liabilities:    
Long-term debt, including current portion 0 0
Total financial liabilities $ 0 $ 0