Employee benefit plan - Schedule of Net Periodic Pension (Benefit) Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 176	$ 201	$ 350	$ 359
Interest cost	89	79	176	147
Effect of foreign exchange rates	(320)	220	25	592
Amortization of prior service cost	(77)	0	(52)	0
Amortization of gain	(4)	(7)	(10)	(15)
Compensation-non-retirement post employment benefits	14	1	18	5
Settlement gain	0	0	0	(7)
Net periodic pension (benefit) cost	$ (122)	$ 494	$ 507	$ 1,081