Retirement and pension plans (Change in projected benefit obligation, plan assets, and funded status of plan) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in the balance sheet:
Net amount recognized in accounts payable and accrued expenses	$ (16,979)	$ (15,722)
Defined benefit plans
Change in benefit obligation:
Projected benefit obligation, beginning of period	15,722	16,493
Service cost	1,026	1,024
Interest cost	594	434
Actuarial losses (gains)	1,473	(46)
Benefit payments	(98)	(98)
Settlements	(1,738)	(2,085)
Projected benefit obligation, end of period	16,979	15,722
Change in plan assets:
Fair value of plan assets, beginning of period	0
Employer contributions	1,836	2,186
Benefit payments	(98)	(98)
Settlements	(1,738)	(2,088)
Fair value of plan assets, end of period	0	0
Funded status, end of period	(16,979)	(15,722)
Amounts recognized in the balance sheet:
Net amount recognized in accounts payable and accrued expenses	(16,979)	(15,722)
Amounts recognized in accumulated other comprehensive loss consist of
Net loss (gain)	226	(517)
Prior service credit	6	4
Net amount recognized	232	(513)
Summary of projected benefit obligation, accumulated benefit obligation and fair value of plan assets:
Projected benefit obligation	16,979	15,722
Accumulated benefit obligation	13,480	13,308
Fair value of plan assets	$ 0	$ 0