Employee Benefit Plans - Components of net periodic benefit cost (Details) - Retirement Income Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period	$ 0	$ 0	$ 0
Interest cost on projected benefit obligation	1,960	1,832	1,932
Expected return on plan assets	(2,598)	(2,837)	(2,356)
Amortization of net loss	919	824	851
Net periodic benefit plan cost	$ 281	$ (181)	$ 427