Income taxes - Deferred tax assets and liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 46,998	$ 44,565
Outside basis differences for U.S. partnerships	7,941	8,656
Operating lease liabilities	4,076	4,695
Deferred revenue	800	782
Deferred compensation	86	623
State taxes	39	44
Stock options	0	0
Other	2,099	939
Valuation allowance	(46,459)	(41,646)	$ (33,810)	$ (34,990)
Net deferred tax assets	15,580	18,658
Deferred tax liabilities:
Property and equipment	(5,729)	(6,943)
Convertible Notes	(3,403)	(4,366)
Operating lease right-of-use assets	(2,888)	(3,348)
Intangible assets	(3,106)	(3,079)
Stock options	(1,438)	(1,915)
Net deferred tax liabilities	(16,564)	(19,651)
Net deferred taxes	$ (984)	$ (993)