Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net periodic benefit income
Interest cost	$ 682	$ 656
Expected return on plan assets	(1,740)	(1,934)
Amortization of prior service credit	(355)	(355)
Amortization of net actuarial losses	806	721
Net periodic benefit income	$ (607)	$ (912)