Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0
Other operating expenses
Interest cost	3,998	5,847
Expected return on plan assets	(9,670)	(9,526)
Amortization of prior service cost (credit)	0	0
Amortization of net loss	4,720	5,220
Total net periodic pension cost	(952)	1,541
OPEB Plan
Personnel costs:
Service cost	159	178
Other operating expenses
Interest cost	893	1,228
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	0	0
Amortization of net loss	470	142
Total net periodic pension cost	$ 1,522	$ 1,548