Revenue - Deferred revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue.
Balance, beginning of the period	$ 35,518	$ 30,416	$ 23,058	$ 29,251
Deferral of revenue	38,867	4,030	55,011	10,320
Recognition of deferred revenue	(16,145)	(17,477)	(19,829)	(22,602)
Balance, end of the period	$ 58,240	$ 16,969	$ 58,240	$ 16,969