Pension and postretirement benefit - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	7,887	4,800	15,774	9,600
Expected return on plan assets	(8,591)	(8,847)	(17,183)	(17,694)
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	5,366	3,911	10,732	7,822
Total net periodic pension cost	4,662	(136)	9,323	(272)
OPEB Plan
Personnel costs:
Service cost	48	121	95	242
Other operating expenses
Interest cost	1,520	983	3,041	1,966
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	(1,106)	0
Total net periodic pension cost	$ 1,015	$ 1,104	$ 2,030	$ 2,208