Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 226,379	$ 219,522
Nonperforming	34	16
Total	226,413	219,538
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	204,010	198,237
Total	204,010	198,237
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	8,103	5,944
Total	8,103	5,944
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	14,266	15,341
Nonperforming	34	16
Total	$ 14,300	$ 15,357