Retirement Plans - Net Periodic Benefit Cost Recognized for Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net pension cost included the following components:
Service cost-benefits earned during the period	$ 260	$ 256	$ 781	$ 767
Interest cost on projected benefit obligation	256	239	769	718
Amortization of net obligation at transition	1		1	1
Amortization of prior service cost	(10)	(14)	(30)	(43)
Recognized net actuarial loss	137	206	412	618
Net periodic pension cost	644	687	1,933	2,061
Company contributions to pension plans	260	270	834	945
Pension plan payouts to participants	$ 260	$ 270	$ 834	$ 945