Retirement and pension plans (Components of net periodic pension expense) (Details) - Defined benefit plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,005	$ 1,026	$ 1,024
Interest cost	566	594	434
Amortization of prior service credit	0	(2)	(2)
Amortization of net loss	158	374	312
Net periodic benefit cost	1,729	1,992	1,768
Settlement loss	537	356	484
Net periodic pension expense	$ 2,266	$ 2,348	$ 2,252