Income taxes - Income tax by jurisdiction (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
U.S. federal:
Current	$ (5)	$ (20)	$ (18)
Deferred	114	115	4,569
Total U.S. federal	109	95	4,551
U.S. state and local:
Current	(142)	(32)	(285)
Deferred	(123)	(35)	1,048
Total U.S. state and local	(265)	(67)	763
Foreign:
Current	(1)	0	(161)
Total foreign	$ (1)	$ 0	(161)
Tax effect on the equity component of the Convertible Notes recorded as additional paid-in capital			$ 5,686