Pension and postretirement benefit - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	7,886	4,800	23,661	14,399
Expected return on plan assets	(8,591)	(8,847)	(25,774)	(26,541)
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	5,367	3,911	16,099	11,733
Total net periodic pension cost	4,662	(136)	13,986	(409)
OPEB Plan
Personnel costs:
Service cost	48	121	143	364
Other operating expenses
Interest cost	1,520	983	4,561	2,948
Expected return on plan assets	0	0	0	0
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	(553)	0	(1,659)	0
Total net periodic pension cost	$ 1,015	$ 1,104	$ 3,045	$ 3,312