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Property, plant and equipment (Details Textual) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Property Plant And Equipment [line items]    
Interest costs capitalised $ 4,382 $ 5,406
Railway [Member]    
Property Plant And Equipment [line items]    
Increase decrease in value of assets   $ 32,000
Percentage of entitys assets   0.80%
Percentage of entity's revenue   11.00%