Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	5,847	7,110	11,694	14,219
Expected return on plan assets	(9,526)	(8,096)	(19,052)	(16,192)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	5,220	5,876	10,440	11,752
Total net periodic pension cost	1,541	4,890	3,082	9,779
OPEB Plan
Personnel costs:
Service cost	178	190	356	380
Other operating expenses
Interest cost	1,228	1,489	2,456	2,977
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	142	0	284	0
Total net periodic pension cost	$ 1,548	$ 1,679	$ 3,096	$ 3,357