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Prepaid expenses (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Prepaid expenses    
Prepaid energy services(b) $ 25,692 $ 26,582
Prepaid insurance 15,794 13,568
Deferred costs of works for taxes 4,855 4,138
Other prepaid expenses 4,174 2,373
Prepaid expenses 50,515 46,661
Classification by maturity:    
Current portion 25,709 20,969
Non-current portion 24,806 25,692
Prepaid expenses $ 50,515 $ 46,661