Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Net periodic benefit income
Interest cost	$ 496	$ 280	$ 1,489	$ 841
Expected return on plan assets	(1,830)	(1,866)	(5,490)	(5,596)
Amortization of prior service credit	(355)	(355)	(1,066)	(1,066)
Amortization of net actuarial losses	568	225	1,705	674
Net periodic benefit income	$ (1,121)	$ (1,716)	$ (3,362)	$ (5,147)