Allowance for Loan Losses - Performing and Nonperforming Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Recorded Investment [Line Items]
Performing	$ 408,407	$ 407,051
Nonperforming	17
Total	408,424	407,051
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	379,005	382,720
Total	379,005	382,720
Real Estate Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	15,209	10,565
Total	15,209	10,565
Consumer and Other [Member]
Financing Receivable, Recorded Investment [Line Items]
Performing	14,193	13,766
Nonperforming	17
Total	$ 14,210	$ 13,766