Fair value of financial instruments - Summary of Fair Value of Financial Liabilities (Details) - Fair Value Measurements, Recurring Basis - USD ($) $ in Thousands	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018
Financial liabilities:
Long-term debt, including current portion	$ 138,865	$ 148,593	$ 150,719
Total financial liabilities	138,865	148,593	150,719
Current portion of long-term debt	12,568	10,259	9,861
Level 1
Financial liabilities:
Long-term debt, including current portion	0	0	0
Total financial liabilities	0	0	0
Level 2
Financial liabilities:
Long-term debt, including current portion	138,865	148,593	150,719
Total financial liabilities	138,865	148,593	150,719
Level 3
Financial liabilities:
Long-term debt, including current portion	0	0	0
Total financial liabilities	$ 0	$ 0	$ 0