Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Net periodic benefit income
Interest cost	$ 280	$ 242
Expected return on plan assets	(1,866)	(1,875)
Amortization of prior service credit	(355)	(355)
Amortization of net actuarial losses	225	407
Net periodic benefit income	$ (1,716)	$ (1,581)