Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 218,650	$ 219,522
Nonperforming	20	16
Total	218,670	219,538
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	198,490	198,237
Total	198,490	198,237
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	5,358	5,944
Total	5,358	5,944
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	14,802	15,341
Nonperforming	20	16
Total	$ 14,822	$ 15,357