Defined benefit pension obligation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined benefit pension obligation
Employer service cost	$ 760	$ 365	$ 2,056	$ 503
Interest cost	57	29	155	39
Expected return on plan assets	(43)	(23)	(116)	(31)
Amortization of prior service cost	12		37
Amortization of actuarial losses	34		102
Net periodic benefit cost	820	$ 371	2,234	$ 511
Expected contributions			2,100
Accrued pension obligation	$ 7,300		$ 7,300		$ 7,100