Income taxes - Components of Provision for Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current
Federal	$ (71)	$ (49)	$ (144)
State	(3)	36	3
Foreign	(1,854)	(1,634)	363
Provision for income tax, current	(1,928)	(1,647)	222
Deferred
Federal	0	42	4
State	0	0	0
Foreign	295	276	(1,135)
Total provision	$ (1,633)	$ (1,329)	$ (909)