Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0
Other operating expenses
Interest cost	5,847	7,110
Expected return on plan assets	(9,526)	(8,096)
Amortization of prior service cost (credit)	0	0
Amortization of net loss	5,220	5,876
Total net periodic pension cost	1,541	4,890
OPEB Plan
Personnel costs:
Service cost	178	190
Other operating expenses
Interest cost	1,228	1,489
Expected return on plan assets	0	0
Amortization of prior service cost (credit)	0	0
Amortization of net loss	142	0
Total net periodic pension cost	$ 1,548	$ 1,679