Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	4,800	3,998	9,600	7,996
Expected return on plan assets	(8,847)	(9,670)	(17,694)	(19,341)
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	3,911	4,720	7,822	9,440
Total net periodic pension cost	(136)	(952)	(272)	(1,905)
OPEB Plan
Personnel costs:
Service cost	121	160	242	320
Other operating expenses
Interest cost	983	893	1,966	1,785
Expected return on plan assets	0	0	0	0
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	0	469	0	939
Total net periodic pension cost	$ 1,104	$ 1,522	$ 2,208	$ 3,044