Employee Benefit Plans - Summary of components of net periodic benefit cost (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Amortization of net loss	$ (804)
Net periodic benefit plan cost	804
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period
Interest cost on projected benefit obligation	1,946	1,741	1,869
Expected return on plan assets	(2,240)	(2,043)	(1,846)
Amortization of net loss	531	784	667
Net periodic benefit plan cost	$ 237	$ 482	$ 690