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, 2019	Dec. 31, 2018	Dec. 31, 2017
Net periodic plan cost included the following components:
Service cost	$ 14	$ 15	$ 21
Interest cost	22	20	23
Amortization of prior service cost	0	0	3
Amortization of gain	(2)	0	0
Net periodic postretirement cost	$ 34	$ 35	$ 47