Employee Benefit Plans - Summary of components of net periodic benefit cost (Details 2) - Retirement Income Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period
Interest cost on projected benefit obligation	2,006	1,898	1,946
Expected return on plan assets	(2,131)	(2,259)	(2,240)
Amortization of net loss	799	790	531
Net periodic benefit plan cost	$ 674	$ 429	$ 237