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Salaries and social security expenses (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
employee
Dec. 31, 2016
USD ($)
employee
Dec. 31, 2015
USD ($)
employee
Analysis of income and expense [abstract]      
Wages and salaries $ 132,025 $ 117,423 $ 104,216
Social security costs 30,558 28,849 23,111
Equity-settled share-based compensation 5,552 4,796 4,396
Salaries and social security expenses $ 168,135 $ 151,068 $ 131,723
Number of employees | employee 7,790 8,326 8,089
Wages and salaries capitalized in property, plant and equipment $ 41,172 $ 28,475 $ 16,708