Employee Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 299	$ 274	$ 897	$ 822
Interest	314	323	942	970
Expected return on plan assets	(470)	(410)	(1,410)	(1,230)
Recognized prior service cost (benefit)	(26)	(26)	(78)	(78)
Recognized net actuarial losses	158	184	473	553
Net periodic benefit cost	275	345	824	1,037
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	381	355	1,144	1,066
Interest	267	231	803	693
Recognized prior service cost (benefit)	29	29	86	87
Recognized net actuarial losses	129	84	386	252
Net periodic benefit cost	$ 806	$ 699	$ 2,419	$ 2,098