Loans - Schedule of Loans Not Rated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 190,173	$ 178,041
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	886	914
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	2,870	811
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	163,255	156,497
Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	3,862	3,513
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	19,300	16,306
Performing [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	189,228	177,095
Performing [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	886	914
Performing [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	2,870	811
Performing [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	162,328	155,608
Performing [Member] | Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	3,862	3,513
Performing [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	19,282	16,249
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	945	946
Nonperforming [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	927	889
Nonperforming [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 18	$ 57