Directors Retirement Plan - Components of Net Periodic Plan Cost for Retirement Plan (Detail) - Director's Retirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net periodic plan cost included the following components:
Service cost	$ 21	$ 19	$ 19
Interest cost	23	26	46
Amortization of prior service cost	3	12	13
Net periodic postretirement cost	$ 47	$ 57	$ 78