Defined benefit pension obligation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined benefit pension obligation
Employer service cost	$ 680	$ 120	$ 1,296	$ 120
Interest cost	52	10	98	10
Expected return on plan assets	(38)	(8)	(73)	(8)
Amortization of prior service cost	13	18	25	18
Amortization of actuarial losses	34		68
Net periodic benefit cost	741	$ 140	1,414	$ 140
Expected contributions			2,100
Accrued pension obligation	$ 7,400		$ 7,400