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Financial risk management - Swaps (Details) - Interest rate swap
$ in Thousands
Dec. 31, 2018
USD ($)
Dec. 31, 2018
GBP (£)
Jun. 30, 2018
USD ($)
Jun. 30, 2018
GBP (£)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
GBP (£)
Financial risk management activities            
Current hedged principal value of loan outstanding | $ $ 150,000   $ 150,000   $ 225,000  
Fair value of interest rate swaps, asset | £   £ 2,504,000   £ 4,490,000   £ 907,000
Long position            
Financial risk management activities            
Rate paid 2.032% 2.032% 2.032% 2.032% 2.032% 2.032%