Pension Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic benefit cost
Service cost	$ 76	$ 68	$ 228	$ 204
Interest cost	55	50	165	150
Expected return on assets	(111)	(90)	(333)	(270)
Amortization of prior service cost and net loss	(10)	(6)	(30)	(18)
Pension expense	$ 10	$ 22	$ 30	$ 66