Retirement Plans - Net Periodic Benefit Cost Recognized for Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net pension cost included the following components:
Service cost-benefits earned during the period	$ 235	$ 260	$ 706	$ 781
Interest cost on projected benefit obligation	248	256	743	769
Amortization of net obligation at transition		1	1	1
Amortization of prior service cost	(3)	(10)	(8)	(30)
Recognized net actuarial loss	98	137	292	412
Net periodic pension cost	578	644	1,734	1,933
Company contributions to pension plans	267	260	856	834
Pension plan payouts to participants	$ 267	$ 260	$ 856	$ 834