Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Net periodic benefit income
Interest cost	$ 435	$ 496	$ 871	$ 993
Expected return on plan assets	(2,099)	(1,830)	(4,197)	(3,660)
Amortization of prior service credit	(355)	(355)	(711)	(711)
Amortization of net actuarial losses	334	568	667	1,137
Net periodic benefit income	$ (1,685)	$ (1,121)	$ (3,370)	$ (2,241)