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Long-term debt - Additional Information (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Debt Instrument  
Long-term debt at fixed rates $ 40,434
Long-term debt at adjustable rates $ 9,300
Interest rate description three-month Libor plus 1.21% to plus 1.57%
Minimum  
Debt Instrument  
Interest rate (as a percent) 1.21%
Maximum  
Debt Instrument  
Interest rate (as a percent) 1.57%