Employee Benefits - Components of Net Periodic Benefit Cost (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
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 299	$ 274	$ 598	$ 548
Interest	314	323	628	647
Expected return on plan assets	(470)	(410)	(940)	(820)
Recognized prior service cost (benefit)	(26)	(26)	(52)	(52)
Recognized net actuarial losses	158	184	316	368
Net periodic benefit cost	275	345	550	691
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	381	355	763	711
Interest	267	231	534	462
Recognized prior service cost (benefit)	29	29	58	58
Recognized net actuarial losses	129	84	258	168
Net periodic benefit cost	$ 806	$ 699	$ 1,613	$ 1,399