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Revenue - Deferred revenue (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue.      
Balance, beginning of the period $ 29,251 $ 22,584 $ 18,145
Deferral of revenue 17,862 24,403 17,284
Recognition of deferred revenue (24,055) (17,736) (12,845)
Balance, end of the period $ 23,058 $ 29,251 $ 22,584