Postretirement benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net periodic benefit income
Interest cost	$ 682	$ 656	$ 2,046	$ 1,969
Expected return on plan assets	(1,740)	(1,934)	(5,218)	(5,802)
Amortization of prior service credit	(355)	(355)	(1,066)	(1,066)
Amortization of net actuarial losses	806	721	2,417	2,163
Net periodic benefit income	$ (607)	$ (912)	$ (1,821)	$ (2,736)