Employee Benefit Plans - Summary of components of net periodic benefit cost (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Amortization of net loss	$ (484)
Net periodic benefit plan cost	484
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period
Interest cost on projected benefit obligation	1,741	1,869	1,916
Expected return on plan assets	(2,043)	(1,846)	(1,831)
Amortization of net loss	784	667	463
Net periodic benefit plan cost	$ 482	$ 690	$ 548