Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 412,807	$ 407,201
Nonperforming	34
Total	412,841	407,201
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	387,228	378,999
Total	387,228	378,999
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	10,787	10,372
Total	10,787	10,372
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	14,792	17,830
Nonperforming	34
Total	$ 14,826	$ 17,830