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Disaggregated revenue - Deferred revenue (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Revenue From Contracts With Customers [Abstract]    
Current contract liabilities $ 17,997 $ 12,687
Decrease from revenue recognized (59,295) (40,707)
Increase due to amounts invoiced 69,341 45,406
Foreign exchange and other movements 288 611
Ending balance $ 28,331 $ 17,997