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Trade and other receivables - Contract assets (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
Subclassifications of assets, liabilities and equities [abstract]  
At January 1, 2018 $ 1,004
Additions to contract assets 9,030
Invoiced during the year (940)
At December 31, 2018 $ 9,094