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Revenue - Deferred revenue (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Revenue.    
Balance, beginning of the period $ 23,058 $ 29,251
Deferral of revenue 11,264 9,799
Recognition of deferred revenue (12,180) (4,944)
Balance, end of the period $ 22,142 $ 34,106