Employee Benefits - Components of Net Periodic Pension Cost and Other Changes in Plan Assets and Benefit (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Components of Net Periodic Pension Cost
Service cost-benefits earned during the period	$ 708	$ 727	$ 1,416	$ 1,454
Interest cost on projected benefit obligation	996	1,076	1,992	2,152
Expected return on plan assets	(1,565)	(1,404)	(3,130)	(2,808)
Amortization of prior service cost (credit)	(34)	(33)	(68)	(65)
Recognized net actuarial loss	588	571	1,176	1,141
Net Periodic Pension Expense	$ 693	$ 937	$ 1,386	$ 1,874