Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 336	$ 258	$ 1,008	$ 775
Interest	394	367	1,182	1,100
Expected return on plan assets	(688)	(636)	(2,064)	(1,907)
Recognized prior service cost (benefit)	(26)	(26)	(78)	(78)
Recognized net actuarial losses	203	3	610	9
Net periodic benefit (credit) cost	219	(34)	658	(101)
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	397	389	1,191	1,166
Interest	341	331	1,023	994
Recognized prior service cost (benefit)	29	29	87	86
Recognized net actuarial losses	150	88	449	266
Net periodic benefit (credit) cost	$ 917	$ 837	$ 2,750	$ 2,512