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Disaggregated revenue - Unbilled trade receivable (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Revenue From Contracts With Customers [Abstract]    
Beginning balance $ 736 $ 372
Decrease from transfers to trade receivables (736) (372)
Increase from revenue recognized 706 736
Ending balance $ 706 $ 736