Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 318	$ 336	$ 636	$ 672
Interest	340	394	680	788
Expected return on plan assets	(694)	(688)	(1,388)	(1,376)
Recognized prior service cost (benefit)	(26)	(26)	(52)	(52)
Recognized net actuarial losses	200	204	400	407
Net periodic benefit (credit) cost	138	220	276	439
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	455	397	910	794
Interest	334	341	668	682
Expected return on plan assets	0		0
Recognized prior service cost (benefit)	29	29	58	58
Recognized net actuarial losses	200	149	400	299
Net periodic benefit (credit) cost	$ 1,018	$ 916	$ 2,036	$ 1,833