Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plans
Personnel costs:
Service cost	$ 0	$ 0	$ 0	$ 0
Other operating expenses
Interest cost	7,109	6,373	21,329	19,119
Expected return on plan assets	(8,096)	(10,060)	(24,288)	(30,180)
Amortization of prior service cost (credit)	0	0	0	0
Amortization of net loss	5,877	5,065	17,629	15,195
Total net periodic pension cost	4,890	1,378	14,670	4,134
OPEB Plan
Personnel costs:
Service cost	190	257	570	771
Other operating expenses
Interest cost	1,489	1,390	4,466	4,171
Expected return on plan assets	0	0	0	0
Amortization of prior service cost (credit)	0	(868)	0	(2,603)
Amortization of net loss	0	321	0	962
Total net periodic pension cost	$ 1,679	$ 1,100	$ 5,036	$ 3,301