Retirement Plans - Net Periodic Benefit Cost Recognized for Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net pension cost included the following components:
Service cost-benefits earned during the period	$ 243	$ 235	$ 486	$ 470
Interest cost on projected benefit obligation	237	248	475	496
Amortization of net obligation at transition		1	1	1
Amortization of prior service cost	(13)	(3)	(27)	(6)
Recognized net actuarial loss	127	97	254	195
Net periodic pension cost	594	578	1,189	1,156
Company contributions to pension plans	287	329	554	588
Pension plan payouts to participants	$ 287	$ 329	$ 554	$ 588