Employee Benefits - Components of Net Periodic Benefit Cost (Credit) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 318	$ 336	$ 954	$ 1,008
Interest	340	394	1,020	1,182
Expected return on plan assets	(694)	(688)	(2,082)	(2,064)
Recognized prior service cost (benefit)	(26)	(26)	(78)	(78)
Recognized net actuarial losses	200	203	600	610
Net periodic benefit (credit) cost	138	219	414	658
Supplemental Insurance/ Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	455	397	1,366	1,191
Interest	334	341	1,002	1,023
Recognized prior service cost (benefit)	29	29	87	87
Recognized net actuarial losses	200	150	600	449
Net periodic benefit (credit) cost	$ 1,018	$ 917	$ 3,055	$ 2,750