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Financial risk management - Summary of Capital Risk Management (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Disclosure of detailed information about financial instruments [abstract]        
Total debt $ 817,958 $ 635,396 $ 723,339  
Total equity 645,131 671,673 $ 527,419 $ 777,227
TOTAL SHAREHOLDERS EQUITY AND LIABILITIES $ 1,463,089 $ 1,307,069    
Gearing ratio 56.00% 49.00%