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, 2018	Dec. 31, 2017	Dec. 31, 2016
Net periodic plan cost included the following components:
Service cost	$ 15	$ 21	$ 19
Interest cost	20	23	26
Amortization of prior service cost	0	3	12
Net periodic postretirement cost	$ 35	$ 47	$ 57