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Financial risk management - Hedging activities (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
$ / £
Jun. 30, 2018
USD ($)
$ / £
Dec. 31, 2017
USD ($)
$ / £
Financial risk management activities      
Closing exchange rate | $ / £ 1.2790 1.3194 1.3513
US dollar      
Financial risk management activities      
Net debt $ 439,600 $ 521,500 $ 538,800
Hedged future USD revenues (338,046) (307,019) (296,369)
Unhedged USD borrowings 101,554 214,481 242,431
Borrowings | US dollar      
Financial risk management activities      
Net debt 650,000 650,000 650,000
Cash and cash equivalents | US dollar      
Financial risk management activities      
Net debt $ (210,400) $ (128,500) $ (111,200)