Fair Value Measurements - Schedule of Balances of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities:
Available-for-sale securities	$ 81,215	$ 77,153	[1]
Mortgage servicing rights	981	999	[1]
Fair Value, Measurements, Recurring
Available-for-sale securities:
Available-for-sale securities	81,215	77,153
Mortgage servicing rights	981	999
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Available-for-sale securities:
Available-for-sale securities	70,074	70,436
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Available-for-sale securities:
Available-for-sale securities	11,141	6,717
Mortgage servicing rights	981	999
US Government Agencies Agencies and Mortgage Backed Securities | Fair Value, Measurements, Recurring
Available-for-sale securities:
Available-for-sale securities	50,136	49,283
US Government Agencies Agencies and Mortgage Backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Available-for-sale securities:
Available-for-sale securities	50,136	49,283
State and Municipal Obligations | Fair Value, Measurements, Recurring
Available-for-sale securities:
Available-for-sale securities	19,938	21,153
State and Municipal Obligations | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Available-for-sale securities:
Available-for-sale securities	19,938	21,153
Corporate Bonds | Fair Value, Measurements, Recurring
Available-for-sale securities:
Available-for-sale securities	11,141	6,717
Corporate Bonds | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Available-for-sale securities:
Available-for-sale securities	$ 11,141	$ 6,717

[1] Derived from audited December 31, 2017 Consolidated Financial Statements.