Retirement Plans - Net Periodic Benefit Cost Recognized for Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net pension cost included the following components:
Service cost-benefits earned during the period	$ 941	$ 1,042	$ 1,023
Interest cost on projected benefit obligation	991	1,025	957
Amortization of net obligation at transition	2	2	2
Amortization of prior service cost	(12)	(41)	(57)
Recognized net actuarial loss	390	549	823
Net periodic pension cost	$ 2,312	$ 2,577	$ 2,748