Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0
Other operating expenses
Interest cost	7,558	7,887
Expected return on plan assets	(8,594)	(8,591)
Amortization of prior service cost/(credit)	0	0
Amortization of net loss	4,166	5,366
Total net periodic pension cost	3,130	4,662
OPEB Plan
Personnel costs:
Service cost	32	48
Other operating expenses
Interest cost	1,421	1,520
Expected return on plan assets	0	0
Amortization of prior service cost/(credit)	0	0
Amortization of net loss	(548)	(553)
Total net periodic pension cost	$ 905	$ 1,015