Results for the year - Income tax - Components of income tax (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Consolidated statement of profit and loss      
Current income tax (expense) benefit $ (244,288,000) $ (79,000) $ 336,000
Deferred income tax (expense) benefit (23,107,000) 21,282,000  
Total income tax (expense) benefit reported in the statement of profit and loss $ (267,395,000) $ 21,203,000 336,000
Current income tax benefit for participating in joint taxation group     158,000
Refundable tax credit     $ 178,000