Pensions - Other pension and compensation disclosure tables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in the consolidated balance sheets
Non-current assets	$ 187	$ 0
Non-current liabilities	0	650
Amounts recognized in accumulated other comprehensive income (loss)
Net loss	(11,511)	(12,200)
Prior service cost	0	0
Net transitional obligation	0	0
Total amount recognized in accumulated other comprehensive loss	(11,511)	(12,200)
Information about pension plans
Projected benefit obligation	25,377	28,882	$ 25,465	$ 24,556
Accumulated benefit obligation	25,377	28,882
Fair value of plan assets	25,564	28,232	24,018	$ 24,202
Components of net periodic pension benefit cost
Service cost	0	0	0
Interest cost	705	713	861
Expected return on assets	(1,081)	(997)	(1,121)
Net amortization and deferrals	756	780	615
Net periodic benefit cost	380	496	355
Future benefit payments
2019	629
2020	504
2021	648
2022	549
2023	652
Next five years	5,032
Employers’ contributions	$ 2,304	$ 2,030	$ 2,052