Pensions - Other pension and compensation disclosure tables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized in the consolidated balance sheets
Non-current liabilities	$ 650	$ 1,447
Amounts recognized in accumulated other comprehensive income (loss)
Net loss	(12,200)	(12,583)
Prior service cost	0	0
Net transitional obligation	0	0
Total amount recognized in accumulated other comprehensive loss	(12,200)	(12,583)
Information about pension plans
Projected benefit obligation	28,882	25,465	$ 24,556	$ 27,353
Accumulated benefit obligation	28,882	25,465
Fair value of plan assets	28,232	24,018	24,202	$ 24,968
Components of net periodic pension benefit cost
Service cost	0	0	0
Interest cost	713	861	984
Expected return on assets	(997)	(1,121)	(1,034)
Net amortization and deferrals	780	615	750
Net periodic benefit cost	496	355	700
Future benefit payments
2018	483
2019	668
2020	536
2021	689
2022	583
Next five years	5,112
Employers’ contributions	$ 2,030	$ 2,052	$ 2,199