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Deferred revenue (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
Subclassifications of assets, liabilities and equities [abstract]  
At January 1, 2018 $ 8,082
Additions to deferred revenue 19,353
Revenue recognized during the year (10,754)
At December 31, 2018 $ 16,681