Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	4,800	3,998	14,399	11,994
Expected return on plan assets	(8,847)	(9,670)	(26,541)	(29,011)
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	3,911	4,719	11,733	14,159
Total net periodic pension cost	(136)	(953)	(409)	(2,858)
OPEB Plan
Personnel costs:
Service cost	121	160	364	480
Other operating expenses
Interest cost	983	893	2,948	2,679
Expected return on plan assets	0	0	0	0
Amortization of prior service cost/(credit)	0	0	0	0
Amortization of net loss	0	468	0	1,407
Total net periodic pension cost	$ 1,104	$ 1,521	$ 3,312	$ 4,566