Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net periodic benefit income
Interest cost	$ 478	$ 682	$ 956	$ 1,364
Expected return on plan assets	(1,905)	(1,740)	(3,809)	(3,479)
Amortization of prior service credit	(355)	(355)	(711)	(711)
Amortization of net actuarial losses	575	806	1,150	1,612
Net periodic benefit income	$ (1,207)	$ (607)	$ (2,414)	$ (1,214)