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Property, plant and equipment (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Property Plant And Equipment [line items]    
Interest costs capitalised $ 5,021 $ 4,382
Impairment loss recognised in profit or loss, property, plant and equipment 1,390 5,205
Other property, plant and equipment [member]    
Property Plant And Equipment [line items]    
Impairment loss recognised in profit or loss, property, plant and equipment 0 0
Site Closure Expenses Capitalised $ 12,967 $ 14,104