Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 216,908	$ 201,018
Nonperforming	44	9
Total	216,952	201,027
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	196,706	179,721
Total	196,706	179,721
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	4,355	5,572
Total	4,355	5,572
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	15,847	15,725
Nonperforming	44	9
Total	$ 15,891	$ 15,734