Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net periodic benefit income
Interest cost	$ 478	$ 682
Expected return on plan assets	(1,905)	(1,740)
Amortization of prior service credit	(355)	(355)
Amortization of net actuarial losses	575	806
Net periodic benefit income	$ (1,207)	$ (607)