Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	3,998	5,847	11,994	17,541
Expected return on plan assets	(9,670)	(9,526)	(29,011)	(28,577)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	4,719	5,220	14,159	15,660
Total net periodic pension cost	(953)	1,541	(2,858)	4,624
OPEB Plan
Personnel costs:
Service cost	160	178	480	535
Other operating expenses
Interest cost	893	1,228	2,679	3,684
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	468	142	1,407	426
Total net periodic pension cost	$ 1,521	$ 1,548	$ 4,566	$ 4,645