Defined benefit pension obligation (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($)
Defined benefit pension obligation
Employer service cost	$ 616
Interest cost	46
Expected return on plan assets	(35)
Amortization of prior service cost	12
Amortization of actuarial losses	34
Net periodic benefit cost	673
Expected contributions	2,000
Accrued pension obligation	$ 7,200