Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	7,110	6,373	14,219	12,746
Expected return on plan assets	(8,096)	(10,060)	(16,192)	(20,119)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	5,876	5,064	11,752	10,130
Total net periodic pension cost	4,890	1,377	9,779	2,757
OPEB Plan
Personnel costs:
Service cost	190	257	380	514
Other operating expenses
Interest cost	1,489	1,390	2,977	2,780
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	(868)	0	(1,735)
Amortization of net loss	0	321	0	641
Total net periodic pension cost	$ 1,679	$ 1,100	$ 3,357	$ 2,200