Revenue - Deferred revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Revenue.
Balance, beginning of the period	$ 22,142	$ 34,106	$ 23,058	$ 29,251
Deferral of revenue	21,893	6,248	30,117	10,724
Recognition of deferred revenue	(8,517)	(9,937)	(17,657)	(9,558)
Balance, end of the period	$ 35,518	$ 30,417	$ 35,518	$ 30,417