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Cash and cash equivalents (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Subclassifications of assets, liabilities and equities [abstract]        
Cash at bank and on hand $ 118,358 $ 130,001    
Short-term bank deposits 150,837 28,567    
Cash and cash equivalents $ 269,195 $ 158,568 $ 198,894 $ 113,795