Employee Retirement Plans and Postretirement Benefits - Net periodic pension benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net periodic pension benefit cost
Service cost	$ 272	$ 841
Interest cost	1,211	802	$ 809
Expected return on plan assets	(615)	(665)	(597)
Amortization of actuarial gain and losses	411	478	503
Net periodic pension cost	$ 1,279	$ 1,456	$ 715