Employee benefit plan - Schedule of Benefit Obligation, Funded Status and Accumulated Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation
Balance at beginning of period	$ 3,556	$ 3,913
Service cost	713	674	$ 707
Interest cost	316	247	259
Actuarial gain	(80)	(371)
Effect of foreign exchange rates	149	(710)
Curtailment	(34)	(5)
Benefits paid	(164)	(192)
Balance at end of period	4,456	3,556	$ 3,913
Underfunded status	(4,456)	(3,556)
Accumulated benefit obligation	$ (3,092)	$ (2,439)