Pensions - Other pension and compensation disclosure tables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Amounts recognized in the consolidated balance sheets
Non-current liabilities	$ 1,447	$ 354
Amounts recognized in accumulated other comprehensive income (loss)
Net loss	(12,583)	(9,934)
Prior service cost	0	0
Net transitional obligation	0	0
Total amount recognized in accumulated other comprehensive loss	(12,583)	(9,934)
Information about pension plans
Projected benefit obligation	25,465	24,556	$ 27,353	$ 24,768
Accumulated benefit obligation	25,465	24,556
Fair value of plan assets	24,018	24,202	24,968	$ 23,162
Components of net periodic pension benefit cost
Service cost	0	0	0
Interest cost	861	984	1,092
Expected return on assets	(1,121)	(1,034)	(1,139)
Net amortization and deferrals	615	750	550
Net periodic benefit cost	355	700	503
Future benefit payments
2017	531
2018	440
2019	609
2020	488
2021	627
Next five years	4,253
Employers’ contributions	$ 2,052	$ 2,199	$ 2,527