Loans - Schedule of Loans Not Rated by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 185,915	$ 178,041
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,015	914
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,193	811
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	161,718	156,497
Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	4,092	3,513
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	17,897	16,306
Performing [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	185,289	177,095
Performing [Member] | Commercial [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,015	914
Performing [Member] | Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	1,193	811
Performing [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	161,112	155,608
Performing [Member] | Construction & Land Development [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	4,092	3,513
Performing [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	17,877	16,249
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	626	946
Nonperforming [Member] | Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	606	889
Nonperforming [Member] | Consumer [Member]
Financing Receivable, Modifications [Line Items]
Loans not rated by class of loans	$ 20	$ 57