Pension and postretirement benefit - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0
Other operating expenses
Interest cost	7,887	4,800
Expected return on plan assets	(8,591)	(8,847)
Amortization of prior service cost/(credit)	0	0
Amortization of net loss	5,366	3,911
Total net periodic pension cost	4,662	(136)
OPEB Plan
Personnel costs:
Service cost	48	121
Other operating expenses
Interest cost	1,520	983
Expected return on plan assets	0	0
Amortization of prior service cost/(credit)	0	0
Amortization of net loss	(553)	0
Total net periodic pension cost	$ 1,015	$ 1,104