Pensions - Schedule of changes in the benefit obligation, plan assets and funded status of the plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at beginning of year	$ 25,465	$ 24,556	$ 27,353
Interest cost	713	861	984
Actuarial loss/(gain)	663	5,259	(702)
Benefits paid	(485)	(529)	(1,723)
Foreign currency translation	2,526	(4,682)	(1,356)
Benefit obligation at end of year	28,882	25,465	24,556
Change in plan assets:
Fair value of plan assets at beginning of year	24,018	24,202	24,968
Actual return on plan assets	1,264	3,116	309
Employer contributions	1,009	1,730	1,946
Benefits paid	(485)	(529)	(1,723)
Foreign currency translation	2,426	(4,501)	(1,298)
Fair value of plan assets at end of year	28,232	24,018	24,202
Funded status at end of year	(650)	(1,447)	(354)
Net actuarial (gain)/loss recognized in other comprehensive loss	$ (383)	$ 2,649	$ (727)