Income taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current
Federal	$ (49)	$ (144)	$ (127)
State	36	3	(13)
Foreign	(1,634)	363	(3,925)
Provision for income tax, current	(1,647)	222	(4,065)
Deferred
Federal	42	4	(24)
State	0	0	0
Foreign	276	(1,135)	492
Total provision	$ (1,329)	$ (909)	$ (3,597)