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, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in the balance sheet:
Net amount recognized in accounts payable and accrued expenses	$ (11,955)	$ (16,979)
Defined benefit plans
Change in benefit obligation:
Projected benefit obligation, beginning of period	16,979	15,722
Service cost	1,005	1,026	$ 1,024
Interest cost	566	594	434
Actuarial (gains) losses	(3,719)	1,473
Benefit payments	(71)	(98)
Settlements	(2,805)	(1,738)
Projected benefit obligation, end of period	11,955	16,979	$ 15,722
Change in plan assets:
Fair value of plan assets, beginning of period	0
Employer contributions	2,876	1,836
Benefit payments	(71)	(98)
Settlements	(2,805)	(1,738)
Fair value of plan assets, end of period	0	0
Funded status, end of period	(11,955)	(16,979)
Amounts recognized in the balance sheet:
Net amount recognized in accounts payable and accrued expenses	(11,955)	(16,979)
Amounts recognized in accumulated other comprehensive loss consist of
Net loss (gain)	(4,188)	226
Prior service credit	6	6
Net amount recognized	(4,182)	232
Summary of projected benefit obligation, accumulated benefit obligation and fair value of plan assets:
Projected benefit obligation	11,955	16,979
Accumulated benefit obligation	10,666	13,480
Fair value of plan assets	$ 0	$ 0