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Financial risk management - Hedging activities (Details)
$ in Thousands
Dec. 31, 2019
USD ($)
$ / £
Jun. 30, 2019
USD ($)
$ / £
Dec. 31, 2018
USD ($)
$ / £
Financial risk management activities      
Closing exchange rate | $ / £ 1.3200 1.2718 1.2790
US dollar      
Financial risk management activities      
Net debt $ 591,108 $ 341,162 $ 439,600
Hedged future USD revenues (199,137) (211,153) (338,046)
Unhedged USD borrowings 391,971 130,009 101,554
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 $ (58,892) $ (308,838) $ (210,400)