Employee benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	19,199	15,993	23,389
Expected return on plan assets	(35,388)	(38,679)	(38,104)
Recognized net actuarial loss	15,644	18,876	20,880
Net periodic benefit cost	545	3,810	(6,165)
Other Adjustments	0	0	0
Total net periodic pension cost	(545)	(3,810)	6,165
OPEB Plan
Personnel costs:
Service cost	485	642	713
Other operating expenses
Interest cost	3,931	3,573	4,913
Expected return on plan assets	0	0	0
Recognized net actuarial loss	0	1,873	567
Net periodic benefit cost	(4,416)	(6,088)	(6,193)
Other Adjustments	60	0	0
Total net periodic pension cost	$ 4,476	$ 6,088	$ 6,193