Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	7,558	7,886	22,675	23,661
Expected return on plan assets	(8,594)	(8,591)	(25,782)	(25,774)
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	4,166	5,367	12,498	16,099
Total net periodic pension cost	3,130	4,662	9,391	13,986
OPEB Plan
Personnel costs:
Service cost	32	48	95	143
Other operating expenses
Interest cost	1,421	1,520	4,264	4,561
Expected return on plan assets	0	0	0	0
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	(548)	(553)	(1,644)	(1,659)
Total net periodic pension cost	$ 905	$ 1,015	$ 2,715	$ 3,045