Revenue - Deferred revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue.
Balance, beginning of the period	$ 30,416	$ 14,080	$ 29,251	$ 22,584
Deferral of revenue	4,030	17,254	10,320	20,887
Recognition of deferred revenue	(17,477)	(3,893)	(22,602)	(16,030)
Balance, ending of the period	$ 16,969	$ 27,441	$ 16,969	$ 27,441