Retirement Plans - Net Periodic Benefit Cost Recognized for Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net pension cost included the following components:
Service cost-benefits earned during the period	$ 1,042	$ 1,023	$ 652
Interest cost on projected benefit obligation	1,025	957	739
Amortization of net obligation at transition	2	2	2
Amortization of prior service cost	(41)	(57)	138
Recognized net actuarial loss	549	823	16
Net periodic pension cost	$ 2,577	$ 2,748	$ 1,547