Income taxes - Income tax by jurisdiction (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S. federal:
Current	$ 18	$ (6)	$ 55
Deferred	(4,569)	(2,787)	345
Total U.S. federal	(4,551)	(2,793)	400
U.S. state and local:
Current	285	503	69
Deferred	(1,048)	212	(42)
Total U.S. state and local	(763)	$ 715	$ 27
Foreign:
Current	161
Total foreign	161
Tax effect on the equity component of the Convertible Notes recorded as additional paid-in capital	$ 5,686