Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 310	$ 318	$ 620	$ 636
Interest	362	340	724	680
Expected return on plan assets	(746)	(694)	(1,492)	(1,388)
Recognized prior service cost (benefit)	(26)	(26)	(52)	(52)
Recognized net actuarial losses	226	200	452	400
Net periodic benefit cost	126	138	252	276
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	395	455	790	910
Interest	345	334	690	668
Recognized prior service cost (benefit)	29	29	58	58
Recognized net actuarial losses	159	200	318	400
Net periodic benefit cost	$ 928	$ 1,018	$ 1,856	$ 2,036