Employee Benefit Plans - Summary of components of net periodic benefit cost (Details) (Retirement Income Plan, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Retirement Income Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned during the period
Interest cost on projected benefit obligation	1,869	1,916	1,893
Expected return on plan assets	(1,846)	(1,831)	(1,720)
Amortization of net loss	667	463	409
Net periodic benefit plan cost	$ 690	$ 548	$ 582