Employee benefit - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	31,548	19,199	15,993
Expected return on plan assets	(34,365)	(35,388)	(38,679)
Recognized net actuarial loss	21,465	15,644	18,876
Net periodic cost (benefit)	18,648	(545)	(3,810)
Other Adjustments	0	0	0
Total cost (benefit)	18,648	(545)	(3,810)
OPEB Plan
Personnel costs:
Service cost	191	485	642
Other operating expenses
Interest cost	6,082	3,931	3,573
Expected return on plan assets	0	0	0
Recognized net actuarial loss	(2,212)	0	1,873
Net periodic cost (benefit)	4,061	4,416	6,088
Other Adjustments	0	60	0
Total cost (benefit)	$ 4,061	$ 4,476	$ 6,088