Employee benefit obligations - Net Periodic Benefit Cost (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($)
Net Periodic Benefit Cost
Company service cost	$ 312
Interest cost	47
Expected return on plan assets	(38)
Amortization of prior service costs	14
Amortization of transition obligation	69
Total net periodic benefit cost	$ 404
Weighted average assumptions:
Discount rate as of December 31	1.00%
Expected long-term rate of return on assets	1.00%
Rate of compensation increase	1.00%
Mortality and disability assumptions	BVG 2010 GT