Loans - Schedule of Loans Not Rated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	$ 175,005	$ 164,028
Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	861	951
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	234	911
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	152,890	144,098
Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	4,334	4,760
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	16,686	13,308
Performing [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	174,401	163,333
Performing [Member] | Commercial [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	861	951
Performing [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	234	911
Performing [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	152,315	143,440
Performing [Member] | Construction & Land Development [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	4,334	4,760
Performing [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	16,657	13,271
Nonperforming [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	604	695
Nonperforming [Member] | Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	575	658
Nonperforming [Member] | Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans not rated by class of loans	$ 29	$ 37