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Significant accounting policies - Revenue recognition - Changes in deferred revenue from contracts with customers (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2019
USD ($)
Changes in deferred revenue from contracts with customers  
Balance at beginning of period $ 5,957
606 adoption adjustment (86)
Deferral of revenue 3,925
Recognition of deferred revenue (4,633)
Balance at end of period $ 5,163