Employee Benefit Plans - Summary of components of net periodic benefit cost (Details 2) - Retirement Income Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period
Interest cost on projected benefit obligation	$ 1,898	$ 1,946	$ 1,741
Expected return on plan assets	(2,259)	(2,240)	(2,043)
Amortization of net loss	790	531	784
Net periodic benefit plan cost	$ 429	$ 237	$ 482