Employee Benefit Plans - Summary of components of net periodic benefit cost (Details 2) - Retirement Income Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period	$ 0	$ 0	$ 0
Interest cost on projected benefit obligation	1,932	2,006	1,898
Expected return on plan assets	(2,356)	(2,131)	(2,259)
Amortization of net loss	851	799	790
Net periodic benefit plan cost	$ 427	$ 674	$ 429