Components of Net Periodic Benefit Cost of Health and Welfare Plan (Detail) - Post-Retirement Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Benefit Cost:
Service cost		$ 5	$ 4
Interest cost	$ 577	639	700
Amortization of past-service liability	(249)	(249)	(249)
Amortization of net actuarial loss	274	326	383
Net periodic benefit cost	$ 602	$ 721	$ 838